Submitted by KEVLAR60442 t3_118d3ig in askscience
For example, the aquatic ape theory claims that hominids began walking upright for ease of wading, hairlessness was conducive to easier swimming, and the aquatic wrinkling nervous reaction was developed as a way of improving grip with undersea rocks. To me, these all seem like very plausible explanations, so I'm curious as to what the more accepted explanations for these adaptations are.
Thank you. This thoroughly helps my understanding.
Adding to this, no evolutionary biologists look at traits in organisms today and explain how / why they evolved by how they are beneficial. To do so is teleogical, explanation by the purpose they serve rather than the process by which they came to be. It also ignores that sometimes traits serve no actual purpose. They can arise to fixation randomly or the trait is vestigial for some other functions that are no longer relevant, but now serve a different purpose. A good example is that, in the case of human hairlessness, the reason why is still actually a pretty hotly debated subject because we don't really know.
Generally speaking, in order to confirm a trait's purpose and evolution you have to study the impact on fitness when you remove that trait and you have to use nearest relatives or most common ancestors to show how the trait evolved in the model species in question.
For anyone like me that doesn't know, 'teleological' means "Relating to or involving the explanation of phenomena in terms of the purpose they serve rather than of the cause by which they arise."
Sexual selection complicates attempts to explain certain evolutionary changes - sometimes, a trait just becomes more attractive to the other gender and, while that trait may represent an underlying superior fitness (beyond the obvious “can have more babies because I’m more fuckable than others”) it also may not. It’s possible that early humans just decided they weren’t down with hairy boning so much.
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Not always. There's a great example of an evolutionary study on a bird species that had incredibly long tails. Like, tails that were so long that they often interfered with flight and made the bird significantly more likely to be caught by predators.
However, the females preferred males with longer tails. So, what essentially happened is that the male birds continued to grow their tails as long as they possibly could until they hit a sort of critical threshold of being maximally attractive for females, but juuuust short enough that it didn't completely hinder their ability to get away from predators and fly.
Researches assayed this by taking feathers and artificially elongating certain male bird's tails (basically bird hair extensions). They noted that these doctored birds had significantly higher mating rates than other birds, but on the flip side, they also got caught (and killed) by predators much more often.
There's tons of examples of this throughout nature, where sexual selection essentially overrides the fitness loss for 'deleterious' traits.
They certainly can be, for sure, though sometimes the mechanisms can be blurred - for example, is a sexual preference for greater height in place because taller people are inherently better at something because of the height OR is it simply that people with access to sufficient food and nutrients (through a variety of mechanisms) are taller, thus height is just an indicator, rather than a survival mechanism itself. And that doesn’t even begin to account for sexually selected traits that are theorized to be technically detrimental to day-to-day survival BUT that kind of indicate to potential mates “if I can afford to waste energy on this useless trait, imagine how awesome I am in everything else!” (eg peacock feathers).
In other words, sexual selection can be weird to untangle.
That is very interesting considering how often I hear people explaining attraction for certain attributes connected to better survival of the off spring.
Ultimately, being able to attract the interest of a partner is directly connected to better survival in that offspring that are actually born have a better chance of surviving than those that never have the opportunity....it's just the thing that attracts that partner may not contribute to survival after they are born.
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So much this. I can't believe how many highly educated and smart people, some in biological sciences, can't accept that many traits appear for no reason and don't disappear because they have no reason to. My advisor used to say, "evolution doesn't have to help you to happen; it just can't kill you or make others not want to fuck you".
There are some traits that have no impact on survival or reproduction, but are expressed because the genes for that trait are found very close to the genes for another trait that does impact survival or reproduction. This can result in the first trait being passed to the next generation along with the second trait.
A problem here is how we teach evolution; that traits (and by extension) genes are selected. But the reality is in any given environment only a subset of traits are under active selection pressure. Most genes are free to drift by chance and appear and disappear.
I have somewhere of the order of 20-24k genes. I live in an environment where we estimate that 2000-4000 humans gene show adaptations to settled agriculture and cities. Less than half of human genes are estimated to be house keeping (i.e. required by all cells)
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Can you give some examples of the kinds of explanations that are still accepted in the field? Like, "Here's a trait that [some animal] has. Here's the uncontroversial scientific consensus on how it evolved."?
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This is going to feel a little bit targeted, but it’s important: understanding is a story we tell ourselves that feels satisfying. Explanations being plausible contributes nothing to their truth value. The poster above goes over it briefly, but the correct way to test an aquatic ape hypothesis is to look at the adaptations other animals who are aquatic/nonaquatic have and compare. Looking at our adaptations in a vacuum and trying to find an explanation, even if you aren’t picking and choosing is bound to find just-so explanations that are plausible but nearly certainly wrong.
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Good explanation there.
Losing one's fur is also useful to lose heat, which also fits the persistence hunter theory.
Just looking at a few traits isn't going to give the full picture.
This so much. I was wondering the other day about how humans conquered the coldest climates for millions of years, but never regained any fur to brace themselves against the cold. Until I went for a hike in the snow and quickly had to remove my jacket at - 10C because damn, bodies produce a lot of heat when moving.
Plus once humans left the really warm area we seem go have had figured clothing out. Covering yourself in the fur of the animals you hunt is more efficient then growing that fur yourself.
The evolution of language trumped all other adaptations. Houses and clothing and controlled fire followed soon thereafter. From then on, human evolution cannot be understood without considering culture.
Are there any other hunting mammals that have lost their fur?
This theory relates to persistence hunting in particular. Losing fur would not be useful for any other type of hunting.
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Are there any other hunting mammals that have learned Calculus?
This is why I don't change my rate of speed when a squirrel jumps in front of my car. I am willing to let them learn algebra, but not calculus.
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I know people on the internet normally say this sarcastically, but I mean it with all sincerity. You must be a lot of fun to talk to at parties! That's a really cool and concise delivery of some very interesting information. Thanks, Queen/King
With knowledge like these, OP doesnt go to parties..... parties goes to him.
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Are there any traits that we can say are linked to humans' propensity to swim?
I know that humans (and most apes) have no instinct to swim, but humans do learn to swim and enjoy doing so. We've also been fishing for a long time.
Not as far as I know, though there are populations of humans who dive a lot (the Bajau sea people) who have enlarged spleens and several other differences in their blood. However, these people spend huge amounts of time foraging and diving in the water, and have for thousands of years.
I'm thinking much longer ago than that, if homo habilis tools have been found mixed among fish fossils: https://www.wired.com/2010/06/first-fish-diet/
I don't know how one would go about determining what adaptations allowed early hominids to catch fish.
To be fair, though, catching fish doesn't necessarily mean swimming or diving. With a good spear and some practice, you don't even need to go knee-deep.
The other problem with extensive aquatic behavior in early humans is that basically every body of water in Africa bigger than a puddle probably has at least one Nile crocodile in it. That doesn't preclude aquatic behavior, but certainly discourages it.
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Everything I wrote is against AAH. An idea can be wrong even if we don't 100% know the right answer, because we know that particular answer doesn't fit the data.
Think of it like the boardgame Clue. I may not know the answer, but when my friend guesses Col. Musrard in the library with the candlestick and I have all those cards, I know my friend's answer is wrong.
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One possibility is that bipedalism evolved because it allowed us to free up our hands, allowing us to carry resources. Another is the endurance running hypothesis: that we evolved to become good long distance runners, and this shaped our legs.
I wouldn't assume that just because a trait evolved, like our hands wrinkling in the water, that it must have been evolutionarily adaptive. Sometimes our bodies evolve weird quirks, and there's no actual benefit, but also no cost to our actual fitness. E.g hair graying in our elder years.
>Sometimes our bodies evolve weird quirks, and there's no actual benefit, but also no cost to our actual fitness. E.g hair graying in our elder years.)
This cannot be restated frequently enough here.
As long as a trait doesn't negatively impact an organism's reproductive fitness, it might just be carried around for no good reason whatsoever. That's how evolution works - some changes are good, some are bad, and (probably) the vast majority are neither, requiring no further explanation other than mutations cause fun things to happen.
Evolution is honestly a lot more nuanced than people generally realize. Even deleterious mutations and traits can rise to fixation in a population despite our understanding of fitness models.
Also important to note that Evolution doesn't work towards the Best solution.
Just the... least worst.
Pretty true. To kinda expound on that, it works "up," but it can get stuck on local maxima rather than global maxima. Picture two mountains separated by a valley and one being higher than the other. If a species is 'climbing' the smaller peak of fitness then once it gets there it can theoretically never climb down the valley and start climbing the taller mountain. It will always* be stuck on that smaller peak because Evolution doesn't know how to take short term pain for long term gain. It's effectively a greedy algorithm to borrow from a CS concept.
*As long as conditions stay exactly the same. The adaptive landscape is always changing.
This is believed to be the reason no species ever developed wheels despite them being incredibly efficient. It's simply too large an evolutionary jump.
Aye, might not be a wheel, exactly, but there's at least one insect with interlocking gears in their legs that help them jump. a planthopper! https://www.livescience.com/39577-insects-with-leg-gears-discovered.html
How useful would wheels be without roads?
Also, sorry for the amateurish questions, wouldn't that require a rotating axle which is kinda impossible to develop? Like, everything is connected to the rest of the body. Maybe some weird symbiosis could do it? :)
Well, at least on the microscopic scale rotating joints are a thing, some bacteria and archaea have rotating flagella. See https://en.wikipedia.org/wiki/Flagellum
Exactly. You don’t have to be the best at something, just better than anyone else around you
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The least worst is the same thing as the best.
What you probably meant was "good enough".
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> Even deleterious mutations and traits can rise to fixation in a population
To be fair, it requires a lot for a new mutation to spread through a population. For instance polydactylism, or having more than 5 fingers on a hand, is a dominant trait but despite its advantages most of still only have 5 fingers on a hand because it's really hard for a new trait to spread unless it confers a real evolutionary advantage, meaning those who lack it die and most of the survivors have that trait.
Dominance also has no effect on how common a trait is, with fitness levels being the same, an allele being dominant doesn't make it more present within a population.
Sometimes things we think are weird quirks actually have benefits we just haven't realised.
There's a severe cost for females having babies with older males. It's well known that over a certain age, babies from older men have increased problems.
As the father grows older, the number of mutations in the father's genome increases, leading to an increase in the incidence of congenital malformations in offspring [11, 65]. Older paternal age may be harmful to the offspring's health in terms of genetic mutations, telomere length, and epigenetics
But how do you easily tell or estimate a male's age? Well, what about a signal like graying? Imagine if there was a signal that was almost universal among males, easy to spot at a glance, and a decent general guide to age?
I suspect graying is NOT a "weird quirk". It's just one we didn't realise the benefit of. There could be other good reasons for graying too. Graying may be an "honest signal" like stotting.
https://en.wikipedia.org/wiki/Signalling_theory#Honest_signals
I suspect there are a lot less "weird quirks" than people think; just misunderstood adaptations.
It's advantageous to be able to identify less fit mates, but it's advantageous to look like a fit mate whether you are or not. It's difficult to have honest signals which aren't difficult to fake; you can't stot unless you're fit, you can't make an alarm call only when there are predators around unless you can detect the predator. If grey hair is energetically no more or less favourable than coloured hair then it would be difficult for it to spread as an honest signal of unfitness because it could just as easily be faked.
It seems more likely that grey hair is a signal for something else - age and hence some kind of experience/authority, or a side-effect of something else. Testosterone has a lot of effects, it probably has some that weren't specifically selected for.
>If grey hair is energetically no more or less favourable than coloured hair then it would be difficult for it to spread as an honest signal of unfitness because it could just as easily be faked.
You're right. It's not an honest signal, because it can be faked.
However, while it's advantageous for individuals to fake, it's disadvantageous for the populations that contain individuals that fake.....so there's pressure to fake and pressure not to fake. Overall I would guess the pressure not to fake would outweigh the individual pressure to fake but it is just a guess...
>It seems more likely that grey hair is a signal for something else - age and hence some kind of experience/authority, or a side-effect of something else."
I agree. I showed a possible reason for gray hair, I am sure there are others. ANd in reality the total advantages for grey hair are going to be a SUM of the advantages and disadvantages. Whether that sum is a total positive or negative, who knows; the sign (+ or -) will probably differ depending on whether we're talking individual advantages or population advantages.
Which leads back to my original point; that some of the adaptations dismissed as "quirks" probably aren't.
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The endurance hunting hypothesis is on the same pseudo-science grounds as the aquatic ape hypothesis. Humans evolved in rocky terrain, where it would have been very difficult to track animals. And there is concrete evidence against it too. In the one place where animal remains with evidence of being eaten have been discovered alongside early humans, the bones were mostly adult and fit animals in their prime, not young or old which would be the easiest to catch by endurance hunting.
More likely is that early humans were ambush hunters, waiting in the foliage for an unlucky animal to walk by. It's possible that there were groups of humans that used endurance hunting, possibly for sport rather than survival (this is what the only groups of people who practice it today do).
However we do have several adaptations that are very well-suited to the endurance hunter lifestyle - the ability to sweat (quite rare in animals in general), hairlessness (which allows passive heat radiation as well as more effective sweat evaporation), a large surface-area-to-volume ratio (again, good for surface cooling), an upright stance allowing us to see greater distances than most prey animals, and bipedal locomation, which is not very fast but is extremely energy-efficient. We also have spectacularly well-adjusted physiology for the throwing of projectiles, which somewhat compensates for our lack of speed. Our combined torso and shoulder mobility is unparalleled in the animal kingdom.
The high surface area to volume ratio surprises me. Do we have any data on, say, averages across different species to compare to?
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So how did we get that endurance and running gait? However it evolved, humans are capable of covering longer distances in a day than most mammals, and do it voluntarily. And sled dogs might only be capable of keeping up, or beating us, because we selected them for it
Probably lots of moving about the landscape. Land tenure is something we know precious little about for our ancestors but it's reasonable to assume that covering ground can be advantageous generally.
Also, the image of early hominids running pell-mell after game presupposes some things about the world they lived in. Running down prey would, for instance, likely catch the attention of the local predator guild, who might be just as likely to steal your now-weary prey and kill you too. On the face of it, human cursorial hunting sounds ludicrously dangerous in most circumstances. The endurance hunting guys have no real answer to this.
> Also, the image of early hominids running pell-mell after game
Is not the image of endurance hunting. It's running at a steady, sustainable pace - a jog, really - that is not sustainable for the prey animal, which eventually cannot run more.
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Alpha male?
If you are lying in wait where you know an animal is being driven, why would you decide to chase it to exhaustion in a days-long relay race around its herd, when you could jump out and kill it? So instead of one person wasting days running after a deer, it's...a dozen people wasting days running after one deer?
A days long, pointless, relay race in a big circle around its herd??
And what, the herd does nothing but sit around, and the animal never gets back to them the whole time?
Alpha male???
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> And sled dogs might only be capable of keeping up, or beating us, because we selected them for it
And they do it in the freezing cold, most of our endurance advantage is from not overheating after all.
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Also, there are many ways that things in our biology are interconnected and entantangled for no discernable reason. There are a lot of adaptations that have some good and some bad impacts. As long as the good outweighs the bad, it tends to stay.
For example, there is a drug that suppresses a man's body's ability to produce a particular protein. While that protein is suppressed, the man produces very little sperm. A near perfect male contaceptive. The thing is, in addition to enabling sperm production that protein also contributes to alcohol metabolism. So, alcohol makes the user very ill. There's no rhyme or reason that these two operations would be using the same darn protein, but they are. There are thousands of these overlaps that have developed over the millenia.
This is because there's no plan behind evolution. It's too complex to draw straight lines. It's a random mess.
That's not quite what's going on there. The drug in question worked by inhibiting aldehyde dehydrogenase, which is an enzyme which handles the direct metabolites of alcohol, but which isn't specific to just ethanol metabolism. All sorts of aldehydes are produced and consumed by the human body and need to be dealt with.
ALDH is involved in the conversion of retinol to retinoic acid, which is necessary to produce sperm, but the relationship is more of a general purpose tool having many applications than a bizarre coincidence of evolution.
The idea that humans evolved because of our bums seems reasonable. Our butts allowed us to become good at distance, running that we would out - endurance animals sprinting away during hunting until the prey eventually gave up. Having more calories and then fire to unleash those calories or hypothesized in some circles to be a major change in our development
Or could it not be that the hands wrinkling was a far earlier adaptation in our evolution which we just subsequently didn't lose?
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Biological anthropologist here. To add on to what others have said, another idea is that we're meant to be energetically efficient. We know from lab experiments that bipeds burn about half the calories that same sized quadrupeds do when walking. Standing up meant that we individually need less food to survive and could support larger populations on the same land than a quadruped could.
There's also the Provisioning Hypothesis, which relies on the idea that our ancestors were monogamous and that bipedalism freed up the hands in order to gather food more efficiently. Prehistoric monogamy seems like a big jump, but when you compare us to the other primates, the male/female size differences are pretty mild. They're more like what you see in monogamous primates like gibbons and less like ones that have "harem" social structures like gorillas, where the males are much, much larger than the females. (Honestly, I still think it's a bit of a leap though.) Monogamy meant that males can be reasonably sure that their mate's offspring are also theirs, so it makes sense for the males to help supply the females and children with food from a reproductive success viewpoint. The extra help feeding the offspring would free the females up for having pregnancies that were a little closer together, and even a slight increase in reproductive rates can lead to outcompeting other groups without that adaptation.
My biggest complaint about the AAH is that the archaeological evidence says that humans seemed to take a very long time to figure out that seafood was delicious. We don't have ample evidence of humans or hominins exploiting aquatic resources until the past 100,000 years or so, at which point we were anatomically the same as we are today.
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I did one year of Anthropology back in the mid 90s. (wanted to keep it up but the syllabus turned all philosophy in year 2)
And I have no recollection of this aquatic ape theory. I thought the upright stance was theorised to be based on a migration from forest/jungle to savannah environments.
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That's a curious way to view evolution, like it's reactive to what would make our lives more convenient.
Mutations are random, it's very much human choice/action that determines which traits become dominant within the species
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then why no other adaptations, like webbed fingers?
Well, there's some webbing there. Other primates have it as well, but only the ones that swim. BTW, swimming and being able to hold your breath is an adaptation.
Yeah just look at how humanoid everything else in the ocean is...oh...never mind.
I don't know of any 'more accepted hypothesis' that explains it better than AAT, because, for a lack of a better word, we simply have none. The main reason why we humans are here and why not some gorillas instead of us, are our brains. The only reason why we are so smart is docosahexaeionic acid, DHA, primarily and mostly in human diets because of fish and shelfish, as it is produced by algae in marine environments. We do not see human-like brain development in land-mammals. Any animal, not getting enough DHA and living on land, tends to get 'big', but not smart. The reason for that is that in the land-based food chain there simply is no DHA available for brain growth. For humans, seafood probably was the only constantly available food resource and once we learned to digest and aquire it, our development from other apes diverged. And all of this, just because of our brain, I haven't even mentioned the fact that babies can swim before they learn to walk etc., etc., which clearly shows hominids being semi-aquatic for a long time before settling down.
> Any animal, not getting enough DHA and living on land, tends to get 'big', but not smart.
does this mean we could selectively breed smarter and smarter monkeys (etc) by feeding them extra DHA, over multiple generations?
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> feeding them extra DHA, over multiple generations?
If you have a few hundred thousands years to spare, sure. It’s possible. Anatomically modern humans, genus Homo, as in “could walk down the street in clothes and you probably would not notice” have been around between 100,000 - 250,000 years depending on who defines “anatomically modern.”
But upright walking ape hominids have been around much longer, like a million plus.
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GeriatricHydralisk t1_j9gpv7o wrote
Rather than cover what's already covered elsewhere, it's worth pointing out why the AAH fails:
Getting dragged into the particulars of this or that trait is a mistake, operating on too low of a level. Considring organisms are integrates wholes, and considering trait evolution in a comparative context, AAH makes not a damn bit of sense.
It's also why nearly nobody with a PhD in a relevant field takes it even remotely seriously, and the only exception was a plankton ecologist with no training in anthropology.