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Why do most animals never seem to evolve over millenia?

Why do most animals never seem to evolve over millenia?


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People often say, including those with extensive knowledge in biology, that a certain species of animal will evolve in one way or another:

  1. From changing environments.

  2. Mutations.

  3. Possibly even genetic engineering from human animals.

My question lies in the fact that, aside from the latter option, why haven't any differences in animals'(except humans) markup, morphology, intelligence, DNA, behavior, or any habits changed over thousands or (possibly millions) of years?

A cockroach has had the same behavior it has today more than 10 million years ago, and there have been no advancements in the species in the slightest bit.

It makes you question evolution, because why don't other animals (like cockroaches) have any changes over 10+ million years, yet humans, like me and you somewhat, have, in a relative period of time similar to the linked geological period above, evolved from spear tossing hominids into someone brilliant enough to even ponder this question.

If modern humans are the result of mutations in genes, why has no one species over the course of hundreds of millions of years been fit enough, or advanced mentally as we have, or even in any slightest bit?


How come most animals never seem to evolve over millennia?

The word "seem" in your question should not be disregarded. You seem to assume that cockroaches (or most animals as you say) did not change much the last tens or hundreds of thousands of years. But what do you know about that (no offence here)? Have you actually reviewed many kinds of research that estimate the rate of evolution of different randomly chosen lineages in the past 500,000 years? I think you assume that other species evolved slower than humans rather than know it. And you will certainly put much more importance to the evolution of the gene FoxP2 (involved in language) than to a gene allowing cockroaches to have a better sense of smell. This is a biased view of what is a rate of evolution. It would be much wiser to consider a rate of evolution as something like the number of newly arising mutations that succeeded to get fixed in the population. See Haldane's rate of evolution and the Darwin unit. Please don't make the mistake to think that being smart (or complex) is some kind of the goal of evolution and those that are not smart (or complex) are "less evolved" or that they evolved more slowly.

You also seem to want to point on the evolution of DNA and evolution of habits. I guess you might be appreciative of the evolution of human knowledge and culture. But this is obviously something that does not have to do with genetic evolution but is rather a matter of cognitive capacity. You cannot compare the change of culture and traditions of insects and humans as insects have mostly no traditions.

Now, this is obviously true that different lineages evolve at different rates. Many things influence this rates such as the population size, the mutation rate, the generation time, the selection pressure (which itself might depend on social structure or the rate of environmental change for example). In these terms, I would rather believe of Homo sapiens as a lineage that should have a rather slow evolutionary rate.

Homo sapiens is quite a recent species. And speciation is often linked with phenotypic divergence, with niche competition and niche complementarity and therefore with a high rate of evolution. In these terms, I would believe that humans are a lineage with high evolutionary rate.


aside from the latter option, why haven't any differences in animals'(except humans) markup, morphology, intelligence, DNA, behavior, or any habits changed over thousands or (possibly millions) of years?

What evidence is leading you to that conclusion? For horses, example. (From the talkorigins article):

The first equid was Hyracotherium, a small forest animal of the early Eocene. This little animal (10-20" at the shoulder) looked nothing at all like a horse. It had a "doggish" look with an arched back, short neck, short snout, short legs, and long tail. It browsed on fruit and fairly soft foliage, and probably scampered from thicket to thicket like a modern muntjac deer, only stupider, slower, and not as agile. This famous little equid was once known by the lovely name "Eohippus", meaning "dawn horse". Some Hyracotherium traits to notice:Legs were flexible and rotatable with all major bones present and unfused. 4 toes on each front foot, 3 on hind feet. Vestiges of 1st (& 2nd, behind) toes still present. Hyracotherium walked on pads; its feet were like a dog's padded feet, except with small "hoofies" on each toe instead of claws. Small brain with especially small frontal lobes. Low-crowned teeth with 3 incisors, 1 canine, 4 distinct premolars and 3 "grinding" molars in each side of each jaw (this is the "primitive mammalian formula" of teeth). The cusps of the molars were slightly connected in low crests. Typical teeth of an omnivorous browser.

So from that, you conclude that the DNA, morphology, and intelligence of horses hasn't changed at all in 50 million years?


This is a tricky question. First, evolution tend to be slow, although there have been recent examples of very fast evolution as well. So for most evolutionary processes, we are not present long enough to see them either happening or see the outcome. Therefore its also hard to say that no evolution is happening - see your cockroach example. How do you know that these animals are the same as 10 Mio years ago? And even if it is like this, it can also mean that these animals fit their niche so good, that there is not much pressure for further adaption.

This can change pretty fast as examples from mites (here a report in the BBC, this is the original publication). Another example of fast evolution (of bigger animals) are the Cichlids in the Lake Victoria, which developed new after the last time the lake dried up completely something like 12.000 years ago. After that, an estimated 300 endemic species developed (see here) which was then reduced by pollution and other environmental problem. The remaining species are evolving again to occupy the free niches (see here).

In the case of the human, we are pretty lucky, that no other intelligent animal has come up so far. They would have fought for the same biological niche and living space with one species eventually dying out. This has, for example, happened to all the other homo species (habilis, erectus, neanderthalensis). As a species, we are quite young (around 200.000 years), so there is something going on. And there is genetic diversion between humans, but still not as much, that we cannot cross each other anymore. And with 7 billion of us now present, it's not that easy for mutations to come through at our reproduction rate.


In response to this part:

If modern humans are the result of mutations in genes, how come no one species over the course of hundreds of millions of years has been fit enough, or advanced mentally like we have, or even in any slightest bit?

All animals are the result of evolution, which includes mutations.

Now, what you should understand is that evolutionary changes have to be selected for, but also must be immediately useful to the organism if they cost more.

There is a long term tendency in our lineage towards increased brain sizes. Animals -> Mammals -> Primates -> Humans. This long term development need not have happened in the first place. In the Jurassic Period, the most successful group of animals were dinosaurs, who in general had small brains.

In addition, our brains require far more calories than the brain of, say, a chimpanzee. Even if you have a lineage that, over the generations, has a tendency towards larger brain sizes, it would also need to be able to hunt or forage more in compensation for the increased caloric needs. If it were not able to do so, a larger brain would actually be a profoundly negative characteristic, a useless drain of energy.

In addition, the benefits of increased intelligence are highly circumstantial. Consider if you gave a cheetah all the brain power of a human. It might then understand that if it dug a hole and placed fake grass over it, it could catch an antelope for less effort than having to stalk and chase it. Less effort means less calories expended and ultimately most of an organism's fitness has to do with how much energy it expends in trying to procure energy (calories).

But lacking opposable thumbs and hands with digits, it would be unlikely to accomplish such a thing. Also, such tasks are more efficient when done by a group, but many animals do not coordinate group-wise as extensively as humans do. So what we have is at least 3 but probably many more things which all have to come together in the same species for them to rise to the top of the food chain like we did:

  • Dextrous limbs (i.e. opposable thumbs and seperated digits)
  • Brains tripled in size (relative to other species of ape)
  • Behavior of extensive group coordination (tribes)

As you can imagine, a species evolving our intelligence and using it to dominate the local ecosystem as extensively as we do is therefore a rarity.


Evolution is an ongoing process; it has no predetermined goal or direction; it never stops. Nothing ever stops because everything is ever-moving, ever-changing.

Is man more intelligent now than few thousand years ago? Has mankind a better understanding of the phenomenal and noumenal realms now than the people who composed the Upanishads, the Brahmanas, the Vedas ~2,500 years ago, which was preceded by hundreds if not thousands years of oral transmission (myths) from generation to generation?


Why Do We Age? A 46-Species Comparison

Why we age is a tricky evolutionary question. A full set of DNA resides in each of our cells, after all, allowing most of them to replicate again and again and again. Why don’t all tissues regenerate forever? Wouldn’t that be evolutionarily advantageous?

Since the early 1950s, evolutionary biologists have come up with a few explanations, all of which boil down to this: As we get older, our fertility declines and our probability of dying — by bus collision, sword fight, disease, whatever — increases. That combination means that the genetic underpinnings of aging, whatever they are, don’t reveal themselves until after we reproduce. To use the lingo of evolutionary biology, they’re not subject to selective pressure. And that means that senescence, as W.D. Hamilton wrote in 1966, “is an inevitable outcome of evolution.”

Today in Nature, evolutionary biologist Owen Jones and his colleagues have published a first-of-its-kind comparison of the aging patterns of humans and 45 other species. For folks (myself included) who tend to have a people-centric view of biology, the paper is a crazy, fun ride. Sure, some species are like us, with fertility waning and mortality skyrocketing over time. But lots of species show different patterns — bizarrely different. Some organisms are the opposite of humans, becoming more likely to reproduce and less likely to die with each passing year. Others show a spike in both fertility and mortality in old age. Still others show no change in fertility or mortality over their entire lifespan.

That diversity will be surprising to most people who work on human demography. “We’re a bit myopic. We think everything must behave in the same way that we do,” says Jones, an assistant professor of biology at the University of Southern Denmark. “But if you go and speak to someone who works on fish or crocodiles, you’d find that they probably wouldn’t be that surprised.”

What’s most interesting to Jones is not only the great diversity across the tree of life, but the patterns hidden within it. His study found, for example, that most vertebrates show similar patterns, whereas plants are far more variable. “You have to then begin to ask yourself, why are these patterns like they are?” he says. “This article is probably asking more questions than it’s answering.”

This sweeping comparison didn’t require particularly high-tech equipment it could probably have been done a decade ago, if not before. But nobody had done it. One challenge is that it required a deep dive into the published literature to a) find the raw data on all of these species, and to b) get in touch with the researchers who conducted the field work to see if they’d be willing to share it.

After rounding up all of that data there was then the problem of standardizing it. Mortality and fertility rates of various organisms can differ by orders of magnitude. What’s more, for some species — like the white mangrove, red-legged frog, and hermit crab — this data comes from defined stages of development rather than across the entire lifespan. Jones got around these obstacles by defining “relative mortality” and “relative fertility” numbers for each species, calculated by dividing fertility or mortality rate at a particular age by the average rate across the organism’s entire lifespan. This allows for easy comparison across species, just by looking at the shapes of the curves.

“That’s what’s so disarming about it,” says David Reznick, a distinguished professor of biology at the University of California, Riverside, who was not involved in the new study. “They’ve come up with a way of putting everything on the same scale, so you can perceive patterns that have never been looked at before.”

The study shows, for example, that most mammals and, importantly, the species that scientists tend to use in the laboratory, such as C. elegans and Drosophila, have shapes like ours. But others are weird, at least from a human-centric view. Here’s a sampling:


1. Being a biology major is challenging.

I often times spend many nights in the library in a given week. My fellow biology majors and I are typically the stressed students you will see in the library at 3 a.m. writing lab reports, studying piles of worn flashcards, drawing horrifically detailed notes for anatomy, or crying over a general chemistry or OChem textbook. Many biology courses demand a student memorize large chunks of information and it takes a lot of studying to make sure our brains actually encode what we are expected to know. If being a biology major was easy for me, I wouldn’t find it rewarding and I would not be pursuing a degree in this field. However, it is a challenge and I find it to be a very rich and adventurous one that is worth any perceived pains, turmoil, or lack of sleep. If it didn’t challenge me, it wouldn’t change me. At the end of the day, biology is the area I’d love be an expert in!


Are there some animals that have stopped evolving?

Some modern animals look just like their long-extinct ancestors. Have these "living fossils" really not changed in millions of years?

The goblin shark is rarely seen, but when it does show up it makes headlines.

That's partly because of its unusual looks. Its pink flesh gives it the appearance of having been skinned, and a flattened, dagger-like snout protrudes from its head. No wonder it's been called the "alien of the deep".

But the goblin shark also evokes our imagination because of its special history. The family it belongs to, the Mitsukurinidae, seems to have barely changed in 125 million years. That means the goblin shark is a "living fossil", an animal that has survived seemingly unchanged for a huge span of time.

A living fossil will look just like a fossilised animal from millions of years ago. This seems to imply that, for these few species, evolution has stopped entirely &ndash as if they evolved to such a peak of perfection that they just don't need to improve any more. But appearances can be deceiving, and there is more to these extreme survivors than meets the eye.

The term "living fossil" was coined by Charles Darwin in On the Origin of Species in 1859, the book in which he first spelled out the theory of evolution. In one section Darwin discussed the platypus and the lungfish, two modern species that belong to an ancient lineage, and still have some of the key features of their fossilised ancestors.

The fish belonged to a group that was thought to have gone extinct 65 million years ago

Darwin wrote that: "these anomalous forms may almost be called living fossils they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition."

At the time the most famous living fossils had not yet been discovered. That would happen in 1938 in South Africa. A natural history curator called Marjorie Courtenay-Latimer realised that a fish she was examining should not have existed.

The fish belonged to a group that was thought to have gone extinct 65 million years ago, during the same cataclysm that wiped out the dinosaurs. It was a coelacanth.

Coelacanths have roots that stretch back 390 million years. They are large, bottom-dwelling fish that can grow up to 2m long. Their fleshy, limb-like fins and dappled scales look as if they've been flecked with blobs of white paint.

Everyone thought it had died with the dinosaurs

There are two known species: the African coelacanth and the Indonesian coelacanth. Together they are the only survivors of the lobe-finned fishes, a group that once dominated the oceans.

"The discovery of the coelacanth gave the term 'living fossil' a lot of currency," says palaeontologist Richard Fortey. "It was a dramatic discovery, as everyone thought it had died with the dinosaurs."

But the real importance of the coelacanth lies in what it can tell us about the evolution of land animals.

Around 400 million years ago, some fish began to walk on land, using their fins as legs. These explorers gave rise to all the 4-limbed land animals, from lizards and frogs to birds and bears.

Coelacanths living 400 million years ago were not identical to the fish that live on in 2015

In 2013 scientists sequenced the genome of the African coelacanth. They found that it is the closest living relative of those first land animals.

But that doesn't make it a true living fossil. A second study, also published in 2013, examined coelacanth fossils and DNA. It found that the two living species are significantly different to their dinosaur-era ancestors, both in their genes and in the design of their bodies.

"The phrase [living fossil] implies that evolution has not acted on the organism over these long timescales," say Chris Amemiya and Mark Robinson of the Benaroya Research Institute in Seattle, Washington, who worked on the coelacanth genome project. "That is clearly shown not to be true for coelacanths."

Quite simply, their skeletons have changed. A second dorsal fin has transformed from spiny to lobed, and they have lost bones around the rim of the mouth and around their scales. Coelacanths living 400 million years ago were not identical to the fish that live on in 2015. So are there other animals that really haven't changed their bodies?

Tadpole shrimps look even more prehistoric than coelacanths. Each one has a carapace that resembles a sequin. This protects a long tail-like abdomen ending in two long, thin appendages that look like antennae.

It seems the key to the tadpole shrimps' survival may be how they reproduce

Tadpole shrimps are found as far apart as China and Scotland, and have survived for 300 million years. That means they survived the Permian extinction, often known as the Great Dying, which wiped out almost every other animal species.

Given that, you might think tadpole shrimps have evolution all figured out. But genetics says otherwise. According to a 2013 analysis, tadpole shrimps have evolved and diversified significantly over millions of years. "There is clear evidence of evolution," says study leader Africa Gómez of the University of Hull in the UK.

In fact it seems the key to the tadpole shrimps' survival may be how they reproduce. A single tadpole shrimp can reproduce without a partner, because they are both male and female.

Tadpole shrimps are self-fertilising hermaphrodites. They have sperm-producing lobes in their ovaries, so they can fertilise their own eggs.

20,000 years ago, northern Europe was covered in an ice cap

"Hermaphroditism might allow organisms to colonise habitats better," says Gómez. "You only require one egg, so it gives them an edge in regions where there has been recent habitat change."

That could have helped them at the end of the last ice age. "20,000 years ago, northern Europe was covered in an ice cap," says Gómez. When the ice melted, it exposed new flood plains, rivers and ponds. "If you're a hermaphrodite you can colonise that relatively quickly."

They are also evolving. Gómez has found that tadpole shrimps in the Sahara reproduce faster than those in Europe, perhaps so they can finish before their puddle dries out in the heat. What's more, "some of the Australian species seem to have evolved to endure higher salinity in the sea water, whereas that would instantly kill some of the European ones," says Gómez.

So it seems we have been misled into thinking that these animals are unchanged. Partly it's our nature. Humans are visual animals, and good at recognising shapes, says Gómez. It is "hard to look beyond that" and see that there might be something different going on 'under the hood'.

Why on earth are they called living fossils?

Some supposed living fossils aren't even as old as we previously thought. For instance, cycad plants are said to have lived alongside the dinosaurs. No doubt some cycads did, but the DNA of modern cycads shows that they only evolved 12 million years ago.

"They have been evolving non-stop and speciating and radiating, so why on earth are they called living fossils?" asks Gómez.

Still, the overall look of each living fossil has stayed more or less the same. So while they are clearly evolving, perhaps they are doing so more slowly than everything else.

Though it might seem that these species have stagnated, they are changing. "The mathematical reality behind evolution is that there has to be a mechanism to keep you the same," says David Polly of Indiana University in Bloomington.

There really is something special about living fossils

Genes are always mutating, and being reshuffled by sex, but that doesn't necessarily mean big changes to the animals carrying them. "Evolution does not move inevitably forwards towards new morphology and new designs," says Fortey.

Since most species do change, there really is something special about living fossils. "That they've stayed roughly the same means there's something quite active keeping them that way," says Polly. "The interesting question is what."

So what is it about coelacanths, ghost sharks and tuataras? Something has allowed their bodies to stay mostly unchanged for hundreds of millions of years.

It may be because they were in the right places at the right times.

Animals can only survive if they have somewhere to live. Mass extinctions destroy many of these habitats, but not all of them. "If the habitat in which these organisms lived came through one of these crises, that carried through the organisms themselves," says Fortey. "They were then free to evolve after the crisis, and so the line wasn't broken."

Cockroaches can live in many places

Habitats can also disappear slowly. "In the geological past there were certain environments that were widespread and common," says Polly. "As we come to the geological present they have become less common, and there are new environments." This explains why many species have been forced to change.

Some have survived by being adaptable. For instance, cockroaches can live in many places, such as crevices, holes, rocks or drains. "They can live on almost anything," says Fortey, and that probably explains why they have lasted so long.

For less adaptable species, it's a question of picking exactly the right spot.

Take the animals known as Lingula, which are found on the sea floor, near the coast, of the Indian Ocean. They look like mussels, but they actually belong to an ancient group called the brachiopods. Their fossil ancestors lived in the inter-tidal habitat, the area between low and high tide, says Fortey.

Some of these survivors were buoyed through these events because they lived in the right place

During the Permian extinction event, the seas became drained of oxygen. This meant creatures living in the deep sea were particularly vulnerable, which helps explain why around 95% of marine species were wiped out. Meanwhile, land animals were killed off in similar numbers by a drier climate and expanding deserts.

But Lingula's ancestors came through unscathed. In the intertidal zone, the water was continuously recycled so lack of oxygen wasn't a problem. "Some of these survivors were buoyed through these events because they lived in the right place," says Fortey.

Beyond where it lives, a species' attributes can help it survive.

"The fact coelacanths taste disgusting could well have helped them stay alive," says Fortey. They look as if they are covered in mucus, and are said to taste waxy and make those who eat them sick to their stomachs.

Horseshoe crabs are also great survivors. The earliest versions show up in the fossil record nearly half a billion years ago. Modern ones have a particularly colourful secret weapon.

Their bright blue blood coagulates when faced with nasty bacteria, preventing infections from going further. Hundreds of thousands of horseshoe crabs are harvested every year by the medical community, because the crucial chemical in their blood can detect contamination in any solution that might come into contact with blood.

The truth is, there is literally no such thing as a "living fossil". All species evolve, even if it's not obvious.

There is one other species that's been proposed to be a living fossil

Gómez thinks we should retire the term altogether. "Darwin never intended it to be used seriously. The term is over-simplifying and leads to people believing that some things haven't evolved, which is so wrong."

Fortey would rather call creatures like coelacanths 'extreme survivors of a lineage'. It's more accurate, but it's not as catchy.

Finally, there is one other species that's been proposed to be a living fossil. That species is the human race. Is it true, as some people have said, that humans have stopped evolving?

The idea is that technological and medical advances have removed the pressure on us to evolve. Modern societies can keep even the weakest alive, by building shelters and developing vaccines against deadly diseases. As a result, our environment is now much easier to survive in, so we may be just evolving culturally, as David Attenborough suggested in a Radio Times interview in 2013.

Even within the last 10,000 years, humans have changed

However, the genetics doesn't support this. Around 40,000 years ago, the human population exploded, and evolution sped up. In 2007, John Hawks of the University of Wisconsin, Madison and his colleagues studied the DNA from 270 individuals and found that human evolution "has recently accelerated by 100-fold".

Similarly, a 2014 study estimated that the most recent common ancestor of all living humans lived around 239,000 years ago. That is much more recent than some estimates, and again suggests that humans have been evolving rapidly.

Even within the last 10,000 years, humans have changed. The existence of blue eyes, and the ability of some adults to drink animal milk that contains lactose, are two examples of recent innovations.

It's harder to say what has happened in the last few hundred years, when technological progress has been fastest, because it's such a short span of time. But if the other living fossils have taught us anything, it's that it should be impossible for humans to stop evolving.


Why do most animals never seem to evolve over millenia? - Biology

Animals that cannot adapt to changing environments are in danger. Photo: Brian Dewey

If we do not reduce our carbon emissions and instead allow global temperatures to rise by 4.5˚C, up to half the animals and plants in some of the world’s most biodiverse areas could go extinct by 2100, according to a new study. In fact, even if we are able to limit global warming to the Paris climate agreement goal of 2˚ C, areas such as the Amazon and the Galapagos could still lose one quarter of their species, say the researchers, who studied the effects of climate change on 80,000 plants and animals in 35 areas. Another study found that local extinctions (when a species goes extinct in a particular area, but still exists elsewhere) are already occurring in 47 percent of the 976 species studied, in every kind of habitat and climatic zone.

With temperatures rising, precipitation patterns changing, and the weather getting less predictable and more extreme, a 2016 study determined that climate change is already significantly disrupting organisms and ecosystems on land and in water. Animals are not only shifting their range and altering the timing of key life stages— they are also exhibiting differences in their sex ratios, tolerance to heat, and in their bodies. Some of these changes may help a species adapt, while others could speed its demise.

Move, Adapt or Die

Animals can react to climate change in only three ways: They can move, adapt or die.

Many animals are moving to higher elevations and latitudes to escape warming temperatures, but climate change may be happening too quickly for most species to outrun it. In any case, moving is not always a simple solution—entering new territory could mean encountering more competition for food, or interacting with unfamiliar species. Some animals, such as the hamster-like American pika, are at the farthest extent of their range. Pikas need the cool moist conditions of the alpine Sierra Nevadas and Western Rockies, but the rocky habitat they require is getting hotter, drier and less snowy. Because they already live so high in the mountains, when their terrain becomes inhabitable, there’s nowhere left to go. Other animals attempting to move to cooler climes may be hemmed in by highways or other manmade structures.

In addition, some impacts of rising temperatures can’t be outrun. Monarch butterflies take their cues from day length and temperature to fly south from Canada to overwinter in Mexico. Lately, the butterflies’ southern migration has been delayed by up to six weeks because warmer than normal temperatures fail to cue them to fly south. Scientists also found that the onset of cooler temperatures in Mexico stimulates the butterflies to return northward to lay their eggs in the spring.

Monarch butterflies in Mexico. Photo: Pablo Leautaud

As temperatures warm, their migrations could fall out of sync with the bloom time of the nectar-producing plants they rely on for food. Logging where they overwinter in Mexico and the dwindling of the milkweed habitat, where they breed and their larvae feed, due to drought, heat and herbicides are additional factors in the monarch’s decline. Its numbers have decreased by 95 percent in the last two decades.

As temperatures rise in the Arctic and sea ice melts, polar bears are also losing their food source they are often unable to find the sea ice they use to hunt seals from, and rest and breed on. Puffins in the Gulf of Maine normally eat white hake and herring, but as oceans warm, those fish are moving farther north. The puffins are trying to feed their young on butterfish instead, but baby puffins are unable to swallow the larger fish, so many are starving to death.

Some Species are Adapting

Some animals, however, seem to be adapting to changing conditions. As spring arrives earlier, insects emerge earlier. Some migrating birds are laying their eggs earlier to match insect availability so their young will have food. Over the past 65 years, the date when female butterflies in southern Australia emerge from their cocoons has shifted 1.6 days earlier per decade as temperatures there have warmed 0.14˚C per decade.

Coral reefs, which are actually colonies of individual animals called polyps, have experienced extensive bleaching as the oceans warm—when overheated, they expel the colorful symbiotic algae that live within them. Scientists studying corals around American Samoa found that many corals in pools of warmer water had not bleached.

A coral reef in American Samoa. Photo: NOAA

When they exposed these corals to even higher temperatures in the lab, they found that just 20 percent of them expelled their algae, whereas 55 percent of corals from cooler pools also exposed to the high heat expelled theirs. And when corals from a cool pool were moved into a hot pool for a year, their heat tolerance improved—only 32.5 percent now ejected their algae. They adapted without any genetic change.

This coral research illustrates the difference between evolution through natural selection over the course of many generations, and adaptation through phenotypic plasticity—the ability of an organism to change its developmental, behavioral and physical features during its lifetime in response to changes in the environment. (“Plasticity” here means flexible or malleable. It has nothing to do with the hydrocarbon-based products that are clogging our landfills and oceans.) The corals living in the hot pools had evolved over many generations as natural selection favored survival of the most heat-tolerant corals and enabled them to reproduce. But the corals from the cool pool exposed to the hotter water were also able to adapt because they had phenotypic plasticity.

How Does Phenotypic Plasticity Work?

When some animals (and plants) encounter the impacts of climate change in their environment, they respond by changing behavior and moving to a cooler area, modifying their physical bodies to better deal with the heat, or altering the timing of certain activities to match changes in the seasons. These “plastic” changes occur because some genes can produce more than one effect when exposed to different environments.

Organic compounds, called methyl groups, attach to DNA and determine gene expression. Photo: Christoph Bock

Epigenetics—how environmental factors cause genes to be switched on or off—bring about phenotypic plasticity mainly through producing organic compounds that attach to DNA or modifying the proteins that DNA is wound around. This determines whether and how a gene will be expressed, but it does not alter the DNA sequence itself in any way. In some cases, these changes can be passed along to the next generation, but epigenetic changes can also be reversed if the environmental stresses are eliminated.

Scientists don’t know whether all species have the capacity for epigenetic responses. For those that do, epigenetic changes could buy them time to evolve genetic adaptations to changing environmental conditions. And over the long term, phenotypic plasticity could become an evolutionary adaptation if the individuals with the genetic capacity for phenotypic plasticity are better suited to the new environment and survive to reproduce more.

“Like any trait, phenotypic plasticity can undergo natural selection,” emailed Dustin Rubinstein, associate professor in Columbia University’s Department of Ecology, Evolution and Environmental Biology. “This means that when there is a benefit to having a plastic response to the environment, this can be favored by natural selection … Some traits (like behaviors) may be more likely to be plastic than others.”

For species that take a long time to mature and reproduce infrequently, epigenetics may give them the flexibility to be able to adapt to rapidly changing conditions. Species with shorter life spans reproduce more frequently, and the rapid succession of generations helps them evolve genetic adaptations through natural selection much more quickly.

Examples of Epigenetic Changes

Guinea pigs from South America normally mate at a temperature of about 5˚C. After keeping the males at 30˚C for two months, scientists conducting one study found evidence of epigenetic changes on at least ten genes linked to modifying body temperature. The guinea pigs’ offspring also showed epigenetic changes, but these were different from those of their fathers. It seems that that the fathers produced their own epigenetic changes in response to the heat, but passed along to their young a different set of “preparedness” changes.

Illustration of a common skate, Woods Hole, MA.
Photo: David Starr Jordan

A population of winter skate fish from the southern Gulf of St. Lawrence have a much smaller body size than other populations of winter skate along the Atlantic coast. Scientists found that these skates had adapted to the gulf’s 10˚C warmer water temperatures by reducing their body size by 45 percent compared with other populations. (Since oxygen content decreases when oceans warm, it is difficult for bigger fish to get enough oxygen.) The scientists detected 3,653 changes in gene expression that reflected changes in body size and some life history and physiology traits. Despite these epigenetic changes, the DNA of these winter skates—which have lived in the southern Gulf of St. Lawrence for 7,000 years—was identical to that of another Atlantic skate population.

When Phenotypic Plasticity is Not Protective

“It is important to not confuse species responses and adaptation as an indicator that everything will be okay,” said ecologist Brett Scheffers, from the University of Florida.

A prime example is the green sea turtle, whose sex is determined by the temperature of the sand around its egg as it develops. Warmer incubation temperatures produce more females.

A green sea turtle hatchling, probably a female. Photo: GreensMPs

Scientists examined turtles around the Great Barrier Reef, a large and important turtle breeding area in the Pacific. They found that turtles from the cooler southern nesting beaches were 65 to 69 percent female, while those from the warmer northern nesting beaches were 87 percent female. In juvenile turtles, females now outnumber males by about 116 to 1. Turtles are so sensitive that if temperatures rise a few degrees Celsius more, certain areas could end up producing only females, eventually resulting in local extinctions.

Meadow voles born in autumn are born with a thicker coat than those born in spring, thanks to environmental cues the mother relays through her hormones while the pup is in the womb. These predictive adaptive responses, believed to be controlled by epigenetics, guide the animal’s metabolism and physiology to enable it to adapt to the environment it will supposedly be born into. But if it’s suited to life in a certain kind of environment, it could end up being maladapted when conditions change—for instance, if winters become warmer.

The brown butterfly from Africa. Photo: Charlesjsharp

Phenotypic plasticity can even limit adaptive evolution. A butterfly from Malawi speeds up its growth and reproduction and lives a short life when it is born at a warm, wet time of year if born in a cool dry season, it leads an inactive long life with delayed reproduction. While the butterfly has a lot of variety in gene expression, scientists have found very little actual gene variation for this plasticity. The butterflies adapted to very specific, predictable and consistent environmental cues. Natural selection furthered these carefully tuned reactions because any deviation from these precise responses would have been maladaptive. Consequently, over time, natural selection eliminated the genetic variation that would have allowed for more plasticity. So, paradoxically, phenotypic plasticity in seasonal habitats may produce species that are specialists in their particular environments, but are then more vulnerable to climate change.

It’s also believed that species in regions with a very consistent climate will have a harder time adapting to climate change. For example, because the tropics have had little climatic variability over thousands of years, it’s thought that tropical species have less diversity in their genes to deal with changing conditions.

Evolution to the Rescue?

Scott Mills, a professor of wildlife biology at the University of Montana, has been researching global patterns of coat color changes in eight species of hares, weasels and foxes. He has found that individuals who turn white in the winter are more common at higher latitudes, but for some animals, the mismatch of their white coats with less snowfall has led to a reduction in their range.

“We know that whether or not an animal is brown in the winter or white in the winter has a very strong genetic component,” said Mills. “And the coat color change trait doesn’t have much plasticity. There doesn’t seem to be any obvious capacity for them to have behavioral plasticity either—to behave so as to reduce mismatch or reduce being killed by the mismatch.” As snowfall decreases, there will be more and more mismatches, so if these species are to survive, they will have to evolve.

Mills’ research identified some populations of these animals with individuals that turn white and others that stay brown in winter. Because these groups have that genetic variability, they have the best chance to adapt, since evolution operates the fastest when there’s ample variation within a population for natural selection to act upon.

Both phenotypic plasticity and evolutionary change are more likely to occur in larger populations of animals and those connected to other populations. A large, diverse group will have more individuals with genes that allow for phenotypic plasticity, which can ultimately be favored by natural selection. In addition, “generalist” species—those that can live in environments with a wide variety of conditions—usually have more variation in their traits that can be inherited.

“One of the biggest discoveries over the last 20 years in biology,” said Mills, “is that meaningful evolutionary changes can happen fast. Evolution isn’t just for fossils—evolution can happen on ecological time scales in five to 10 generations. That’s led to more anticipation that evolutionary change might be able to play a role in rescuing species…With the right work and focus, this can become another tool in the conservation tool kit.”

What Needs to be Done

Human beings rely on biodiversity—the variety of life on Earth—and functioning ecosystems for food, clean water and our health. If other species are unable to adapt to climate change, the consequences for humans could be dire. Society needs to implement strategies to help wildlife adapt to the impacts of climate. This means identifying and protecting zones where species exhibit genetic variability and preserving natural marine and land-based ecosystems.

Wildlife overpass in Singapore. Photo: Benjamin P. Y-H. Lee

It means purposefully increasing connectivity between natural areas, and providing stretches of land that animals can migrate along and to. These measures would enable species to travel to cooler areas and allow for larger, more connected populations that can promote the genetic diversity needed for phenotypic plasticity and adaptive evolution.

The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) just released four reports on biodiversity. Written by more than 550 experts from 100 countries, the reports found that biodiversity is declining in every region of the world, endangering “economies, livelihoods, food security and the quality of life everywhere.” In the words of IPBES chair Robert Watson: “The time for action was yesterday or the day before.”


The four principles of Karl Ernst von Baer

In 1828, von Baer reported, “I have two small embryos preserved in alcohol, that I forgot to label. At present I am unable to determine the genus to which they belong. They may be lizards, small birds, or even mammals.” Figure 1.5 allows us to appreciate his quandary. All vertebrate embryos (fish, reptiles, amphibians, birds, and mammals) begin with a basically similar structure. From his detailed study of chick development and his comparison of chick embryos with the embryos of other vertebrates, von Baer derived four generalizations (now often referred to as “von Baer's laws”), stated here with some vertebrate examples:

Figure 1.5

The similarities and differences between different vertebrate embryos as they proceed through development. They each begin with a basically similar structure, although they acquire this structure at different ages and sizes. As they develop, they become (more. )

The general features of a large group of animals appear earlier in development than do the specialized features of a smaller group. All developing vertebrates appear very similar shortly after gastrulation. It is only later in development that the special features of class, order, and finally species emerge. All vertebrate embryos have gill arches, notochords, spinal cords, and primitive kidneys.

Less general characters are developed from the more general, until finally the most specialized appear. All vertebrates initially have the same type of skin. Only later does the skin develop fish scales, reptilian scales, bird feathers, or the hair, claws, and nails of mammals. Similarly, the early development of the limb is essentially the same in all vertebrates. Only later do the differences between legs, wings, and arms become apparent.

The embryo of a given species, instead of passing through the adult stages of lower animals, departs more and more from them. ¶ The visceral clefts of embryonic birds and mammals do not resemble the gill slits of adult fish in detail. Rather, they resemble the visceral clefts of embryonic fish and other embryonic vertebrates. Whereas fish preserve and elaborate these clefts into true gill slits, mammals convert them into structures such as the eustachian tubes (between the ear and mouth).

Therefore, the early embryo of a higher animal is never like a lower animal, but only like its early embryo. Human embryos never pass through a stage equivalent to an adult fish or bird. Rather, human embryos initially share characteristics in common with fish and avian embryos. Later, the mammalian and other embryos diverge, none of them passing through the stages of the others.

Von Baer also recognized that there is a common pattern to all vertebrate development: the three germ layers give rise to different organs, and this derivation of the organs is constant whether the organism is a fish, a frog, or a chick.

WEBSITE

1.1 The reception of von Baer's principles. The acceptance of von Baer's principles and their interpretation over the past hundred years has varied enormously. Recent evidence suggests that one important researcher in the 1800s even fabricated data when his own theory went against these postulates. http://www.devbio.com/chap01/link0101.shtml


The Most Popular Textbook Example of Punctuated Evolution Has Been Debunked by Researchers

The picture shows the seven species of bryozoans that were used in the debunking.The white line is only 500 micrometers in lenght. Copyright: JoAnn Sanner, The University of Chicago
The most popular textbook example of

Researchers at the University of Oslo have debunked a textbook example about how evolution proceeds during speciation. Renowned paleontologist Stephen Jay Gould fronted the old theory.

Evolutionary biologists have for a long time disagreed on the rate of evolution when new species emerge. Are new species the result of gradual changes – as Charles Darwin suggested – or is evolution speeding up for short periods of time when new species evolve?

World renowned paleontologist Stephen Jay Gould (1941-2002) formulated the theory of punctuated equilibrium together with Niles Eldredge (1943-) in 1972. The theory states that species remain more or less unaltered during their existence, with major evolutionary change happening during rapid events of speciation. As evidence for this view, Gould pointed to the fossil record.

Fossils can tell scientists about what life on Earth looked like in the past. The picture shows two million year old fossils of marine organisms found on an expedition to New Zealand. Credit: Kjetil Lysne Voje/UiO

According to Gould, the fossil record typically show that species do not change significantly after they emerge, and that major changes occurred when new species appeared.

Stephen Jay Gould was one of the twentieth century’s most famous evolutionary biologists and a bestselling popular science writer. Some even claimed that Gould was the foremost biologist of his time – perhaps the greatest since Charles Darwin himself – so his words have carried a lot of weight to this day.

In a new paper from researchers at the University of Oslo, the authors claim to have found several methodological problems in the most famous and well-trusted example supporting the theory of punctuated equilibrium.

“We find no evidence for punctuated evolution in our reanalysis of the most recognized dataset that Gould used to support his theory,” says Kjetil Lysne Voje at UiO’s Center for Ecological and Evolutionary Synthesis (CEES) at the Department of Biosciences.

Textbook example is rejected

Fossils of the bryozoan genus Metrarabdotos – a group of aquatic invertebrates thoroughly investigated by the excellent paleobiologist Alan Cheetham – have been the prime example of punctuated evolution.

Gould called Metrarabdotos “the most brilliantly persuasive, and most meticulously documented, example ever presented for predominant (in this case, exclusive) punctuated equilibrium in a full lineage” (Gould 2002, page 827).

Researcher Kjetil Lysne Voje led the new study on evolution of species within the bryozoan genus Metrarabdotos. Credit: Unni Vik/UiO

“We detected some critical methodological issues in the original work on Metrarabdotos. When we take the methodological issues into account, we do not find any evidence of punctuated evolution in our reanalysis of the Metrarabdotos data,” says Kjetil Lysne Voje.

Bryozoans are so small that scientists have to use an electron microscope to study them in detail, but they form colonies that can be quite large (up to 1 meter). Most bryozoans live in the sea, but there are also many species in fresh water. The bryozoan genus Metrarabdotos has been used as a textbook example in evolutionary biology and paleontology, showing how evolution speeds up when new species form compared to a much slower evolution of already established species.

“But our new results show nothing else than a gradual evolution of the bryozoan species both before, during and after the formation of new species,” emphasizes Voje.

Why is this important?

The idea of ​​fast-track evolution during speciation has been controversial. Critics of the theory of punctuated equilibrium found it difficult to believe that the evolutionary processes leading to new species should be markedly different from the processes that cause already existing species to change.

“Species are continuously evolving and our results support the hypothesis that evolution does not “behave” differently when new species emerge,” says Voje.

The paper with the new results was published in the May issue of The American Naturalist. The authors of the study are Kjetil Lysne Voje, Emanuela Di Martino and Arthur Porto.

Reference: “Revisiting a Landmark Study System: No Evidence for a Punctuated Mode of Evolution in Metrarabdotos” by Kjetil Lysne Voje, Emanuela Di Martino and Arthur Porto, 17 March 2020, The American Naturalist.
DOI: 10.1086/707664


Byte Size Biology

Some microbes are evil minions of Hell (but not all)

Quite a few people think that microbes are evil, disease causing minions of Hell that should be eradicated. Supermarkets are handing out sanitary wipes: wipe the handlebar if you want to live, never mind that 90% of the food in the supermarket is worse for you than anything you may catch off that cart handle. Almost every public space looks like the secret basement level of the CDC, with alcoholic hand sanitizers and posters portraying the horrors of aerosol-borne infections. Microbes are the invisible enemy: you can’t see them, but they are deadly. You can sure kill them with copious amounts of ethanol.

Actually, only a minority of microbes are pathogens. Some eukaryotes are parasitic and disease causing. There is Athlete’s foot (caused by a fungus) amoebal dysentery and other unpleasant experiences. But most are not. Also, most bacteria that live in us or on us are symbiotic and like us for our throwaway proteins, carbohydrates, nice 36.6C temperature, high humidity (armpit or mouth) and other goodies. Yes, some are pathogenic, and some do seem like evil little minions of the Devil. Those have ingenious mechanism which infect, wreak havoc, sometimes kill, and move on. But for every plague bacillus or burger bug out there, there are millions of other kinds of bacteria that really don’t do much, good or bad.

About archaea

There is one group of microbes that have no known pathogens: Archaea. Archaea are… different. An archaeon is as different from a bacterium as either is from a human. Superficially, bacteria and archaea look the same. Both are unicellular. Both do not have well-formed cellular organelles on the level that eukaryotes have. For those two reasons, archaea were thought, for a very long time, to be a type of bacteria. Today, virtually all microbiologists classify archaea in a domain of their own. Archaeal cell membranes are made up of their own unique type of building-blocks (lipids), the type which bacteria do not have, and neither do eukaryotes. Their cell wall is different than bacteria. Many live in extreme conditions: ocean smokers, geysers, hyper-saline lakes, the frozen Tundra, termite guts, cow stomachs and Charlie Sheen’s pants. Actually, the latter may be a bit too extreme even for archaea. Looking at phylogenetic marker genes, such as small subunit ribosomal RNA, (SSUrRNA) archaea indeed cluster as a domain unto themselves.

But of the hundreds of disease-causing microbes or pathogens that we know of, none are archaea. Which is odd. Plenty of disease causing eukaryotes and bacteria, but no archaea? Why is that? In a new paper published in Bioessays, Erin Gill and Fiona Brinkman try to answer this question.

First, Gill and Brinkman examined the most trivial hypothesis: we may just not have discovered archaeal pathogens yet. Their statistical analysis shows that this is possible, but unlikely. Here is the way the authors explain this: about 0.36% of known bacterial species cause disease (585 out of about 151,000 known cultured and uncultured species, a very low-bound estimate). Assuming that the diversity in archaea is about the same, we should have identified a few (the authors estimate .0036 x 4,508 species of archaea = 16) archaeal species which cause disease. This somewhat back-of-the-envelope calculation is a bit rough and laden with assumptions: one, that the diversity among known archaea is the same as among known bacteria. It was recently discovered that there is a huge marine diversity of mesophilic archaea for which we only have metagenomic (fragmented DNA sequence) data. Also, there may be many diseases we know nothing about, simply because our census of life on earth is far less than complete. Some of these archaea (and more of these bacteria) may be pathogens, only many have not been identified as such. Finally, historically, with bacteria, we were biased towards looking for pathogens. Bacteriology started as a medical discipline, and to this day many microbiology departments reside in universities’ medical schools. On the other hand, archaea were studied mostly by environmental microbiologists, who are not looking for pathogens necessarily, but are more interested in biogeochemical cycles and the diversity of life. But its claim does cause us to raise an eyebrow: not even one known archaeal pathogen? OK that’s odd. Quite worth looking into. Although the number of archaea we can examine may be too small.

So what exactly is going on?

Bacteria don’t kill people. Bacteriophages kill people?

A clue may lie in how virulence genes are arranged in the bacterial genome. Virulence genes are genes that code for proteins that let bacteria invade our body, cause disease and evading the immune system and drugs. Many of these genes are recognized as mobile: they can easily jump together from a disease causing strain to a benign strain, causing the latter to now become virulent. In many cases they can jump between different species. The vector that carries those genes is typically a bacterial virus, or bacteriophage. When a virus invades bacteria, it can uptake some of its DNA and incorporate it into its own genome. This DNA may later be deposited in another bacterium, turning a benign strain into a virulent one. The process of moving DNA between bacteria with a virus is called transduction, and viruses may also leave very specific “fingerprints” in transduced DNA.

Generalized transduction. Source: Indian River State College

One might say that pathogenic bacteria are actually a vehicle to help bacteriophages proliferate. Better yet, bacteriophages and bacteria both can be viewed as vehicles to help virulence genes proliferate.

However, as far as we know, bacteriophages do not invade archaea. Archaea do have their own viruses, but those are different from bacteriophages. Archaea are a separate domain of life, and whatever parasitises one domain would be ill fit to parasitise another. After all, viruses that invade eukaryotes are also quite different from bacteriophages. (As an aside, this is what makes bacteriophages such an attractive idea as an anti-bacterial treatment method: after all, if we can inundate the human body with viruses that only infect bacteria, moreover only specific disease-causing bacteria leaving those that we need unharmed, that would make for a great silver bullet. But bacteriophage treatment is a matter for another post.) The differences are in shape, biochemistry and in genomes. There is little to no similarity in the genomic sequences of archaeal viruses and bacteriophages. No bacteriophages are known to infect archaea and vice-versa. That said, we know precious little about the diversity of bacteriophages, and close to nothing about archaeal viruses.

We do know that archaea have a very different cell-wall biochemistry than bacteria, and lack the receptor proteins which bacteriophages use to infect bacteria. So bacteriophages cannot infect archaea, cannot transmit virulence genes, and cannot transmit virulence. Gill and Brinkman present virulence from the bacteriophage’s (or rather the bacteriophage’s genes) point of view: both bacteria and their hosts are vehicles for propagating bacteriophage genes. A rather complex evolutionary mechanism.

But why haven’t archaea developed virulence of their own, independently of bacteria? Wouldn’t archaeal viruses develop a similar mechanisms? The authors claim the reason is that virulence evolution is a rare event. They argue that the evolution of virulence, at least the virus-transmitted secondary type is a multi-step process, and is therefore rare. My take on this argument: yes, it might be true for phage-transmitted virulence, but both bacteria and eukarya have evolved virulence mechanisms independent of viruses, encompassing many diverse mechanism that appear to have evolved independently. Hence, virulence itself is not so rare, even if the gene-island type may be.

All-in-all a thought provoking paper, which was very exciting to read. The authors qualify their hypothesis heavily, knowing that with bacterial, archaeal and their viruses, there are unknown unknowns, as the following bit of poetry illustrates:

The Unknown
As we know,
There are known knowns.
There are things we know we know.
We also know
There are known unknowns.
That is to say
We know there are some things
We do not know.
But there are also unknown unknowns,
The ones we don’t know
We don’t know.

—Donald Rumsfeld, Feb. 12, 2002, Department of Defense news briefing

Gill, E., & Brinkman, F. (2011). The proportional lack of archaeal pathogens: Do viruses/phages hold the key? BioEssays, 33 (4), 248-254 DOI: 10.1002/bies.201000091


Evolution During the Triassic Period

Confusing matters somewhat, the archosaurs of the middle to late Triassic period didn't only give rise to dinosaurs. Isolated populations of these "ruling reptiles" also spawned the very first pterosaurs and crocodiles. For as much as 20 million years, in fact, the part of the Pangean supercontinent corresponding to modern-day South America was thick with two-legged archosaurs, two-legged dinosaurs, and even two-legged crocodiles—and even experienced paleontologists sometimes have trouble distinguishing between the fossil specimens of these three families!

Experts are unsure whether the archosaurs from which the dinosaurs descended coexisted with the therapsids (mammal-like reptiles) of the late Permian period, or whether they appeared on the scene after the Permian/Triassic Extinction Event 250 million years ago, a geologic upheaval that killed about three-quarters of all land-dwelling animals on earth. From the perspective of dinosaur evolution, though, this may be a distinction without a difference. What's clear is that dinosaurs gained the upper hand by the start of the Jurassic period. (By the way, you may be surprised to learn that therapsids spawned the first mammals around the same time, the late Triassic period, as archosaurs spawned the first dinosaurs.)


Why do we love pets? An expert explains.

Ours is a pet-loving culture. Researchers spend a lot of time exploring what has become known as “human-animal interactions,” and the pet industry spends a lot of money promoting what it prefers to call the “human-animal bond.” But that concept might have been laughable a century ago, when animals served a more utilitarian role in our lives. And it was “deeply unfashionable” among scholars as recently as the 1980s, as John Bradshaw writes in his new book, “The Animals Among Us: How Pets Make Us Human.”

Bradshaw, an honorary research fellow at the University of Bristol in England, would know. He was trained as a biologist — one who began by studying animals, not people, and not their relationship. But he says his work on dog and cat behavior led him to conclude that he would never fully understand those topics without also considering how humans think about their animals. In 1990, he and a small group of other researchers who studied pet ownership coined a term for their field: anthrozoology. Today, university students at a few dozen U.S. universities study the topic he helped pioneer.

In his latest book, Bradshaw argues that our fascination with pets is not because they’re useful, nor even because they’re cute, and certainly not because they’ll make us live longer. Instead, he writes, pet-keeping is an intrinsic part of human nature, one rooted deeply in our own species’ evolution. I spoke with him recently about his conclusions.

This interview has been edited for length and clarity.

I receive loads of press releases and read lots of headlines about how pets make us healthy. But the science is quite a bit more fuzzy, right?

There is evidence that interacting with pets does reduce people’s stress, provided the pet is behaving properly. Good interactions do have quite a profound effect, causing changes in oxytocin and in beta endorphins. Those are actual changes going on in the body of somebody who is stroking a friendly dog. So that’s the upside. The downside is that pets, real pets that actually live with people, cause stress and expense and all sorts of other things that can cause arguments within the family. And if you take humanity as a whole, I suspect that those two things kind of balance out. For every paper that says that pets make you live longer or that they make people healthier, many other reports — particularly those that come from medical professionals, who don’t really have a stake in the field — that find no effect or actually negative effects. The reporting bias is in favor of the good ones, so the study that showed that cat owners were usually more depressed than people who don’t have any pets didn’t rate any headlines. So pet-keeping as a habit, averaged out, is probably not having any major effect on health in either direction. If the dog gets people out and about and likes energetic exercise, then there are probably health benefits. But they’re not just going to come as part of the package.

Why is there such a mismatch in public perception about pets as a panacea and the evidence for it?

I think it’s about a puzzling and unusually unique effect pets give to people, which is what I call the trustworthiness effect, which hasn’t received a huge amount of attention in the press, but it has been replicated in studies in several different countries. People with animals, or as simply described as having a friendly dog with them, instantly become more trustworthy in the eyes of the person who’s encountering that person or having that person described to them. I think it actually explains quite a lot — people are believed when they tell nice stories about animals. Whether that applies to news reports as well, I’m just guessing, but I think it’s a reasonable explanation. I think it also explains a lot of the effects of animal-assisted therapy. The magic is actually in making the person with the animal much more approachable. In a senior residence, it’s not simply the seniors who find the visitor a good person to talk to, but the staff finds the visits beneficial as well. It makes the whole place seem a bit more homely. The dog, or whatever animal, is changing people’s perception of the person doing the therapy. This is the trustworthiness factor, and it explains quite a lot of our biases.

What’s the harm if people have mistaken beliefs about pets? Lots of animals need homes.

I’ve spent a lot of my career pursuing the idea of better welfare for household pets, and I can see some potential risks. The one that we’re seeing most is people bypassing the idea that you have to know about these animals. Fifty or 100 years ago, the knowledge of how to look after animals was passed from person to person. Now we are much more insular. And the idea that simply getting a pet is going to make you happy and de-stress you is not going to work if you don’t do the homework about what the animal needs. One trend which I have particular concern about is for flat-faced dogs. People don’t really understand that having a dog that looks very cute is also likely to have breathing difficulties, eye problems and other health issues. I find that quite distressing. We have a lot of knowledge now about how dogs think and how they feel, and yet that knowledge is still not getting through to a particular kind of owner who is just obeying the fashion and their gut instincts. They’re told that this is going to be a really good experience for them, and maybe it is, but it probably won’t be that great an experience for the dog.

Why do we keep getting pets?

Pet-keeping is a human universal, and it’s something that’s been going on for tens of thousands of years. So why do people want to do something which seems completely unproductive?

One answer is that there is this satisfaction — stroking a dog or a cat causes hormones to be released and makes the person doing it feel good. I think you can trace that back to our very ancient history as hairy primates. Grooming one another is the main glue that holds most primate societies together. Now we’ve got other ways of socializing, but somewhere deep in our brains is a need to do this grooming of something that’s hairy, and we can satisfy that by stroking a dog or combing the cat.

We also have to explain why it’s persisted when we’d have more money if we didn’t have pets. I think it used to be adaptive — people who were seen to be good with animals were more accepted by other people in their tribe, and there may have even been some selection for brides and grooms based on affinity with animals. Second, domestication of animals has been a very important aspect of the emergence of what we call civilization. But it’s actually intrinsically improbable, because to domesticate an animal you have to change its genetics. Even nowadays that takes many generations. I think the only way you can account for the separation of domestic animals from their wild ancestors, and the only way they stopped interbreeding, is because the domestic animals, the ones that were slightly tamer, were people’s pets and so were physically and emotionally and culturally separated. So we had the emergence of a domestic dog, which is useful, a domestic cat, which can be useful because it hunts around houses, and goats and sheep that you can herd and milk. Pet-keeping became an advantage, because the societies that were good at it and wanted to do it domesticated animals before other neighboring societies and groups of people.

These days , we spend lots of money to keep pets alive, we send them to spas and we buy them furniture. How did things go from pet-keeping to pet indulgence?