GeriatricHydralisk t1_jct5ayu wrote

False, horses evolved in the Americas first, and precursor species as well as modern horses repeatedly migrated across the land bridge to Eurasia.

Then, about 12,000 years ago, they all died out in America, a long with all the other native Megafauna, right around the time a certain species with tools, fire, and a reputation to eating things into extinction showed up.


GeriatricHydralisk t1_jc1zrn3 wrote

This can be true of animals too - many species depend upon certain prey, micronutrients, temperatures, water parameters, etc. Others can be kept, but can't be induced to breed. Usually it's just lack of knowledge, but getting that knowledge requires a lot of trial and error and frustration.


GeriatricHydralisk t1_jb7ptv0 wrote

The terrestrial version are called Tetrapods (literally "four feet"), which originated about 350 million years ago, with modern descendants being amphibians, reptiles, birds, and mammals (though some have subsequently lost tails like us, or feet like snakes).

However, these limbs are just modifications of pectoral and pelvic fins of our fishy ancestors. The true origin of this body plan is 435 million years ago, with gnathostomes, the jawed vertebrates. This is the origin of the two pairs of fins, which eventually became limbs.


GeriatricHydralisk t1_jb09yqx wrote

The key is that heritability is statistical, especially with a big population.

Imagine a trait like height, which is continuous and polygenic. Very tall people and very short people (absent any endocrine or developmental pathology) will mostly have alleles for tall and short, respectively, while average height people will have a mix. If your parents are very tall and very short, you'll get alleles from both sides and likely wind up average. Conversely, if your parents are both average, there's a slim chance you could inherit mostly tall or short alleles, but chances are you'll be average. If both parents are at a height extreme, though, variability is lower.

So to estimate heritability, you regress your height against the average of your parents' heights. With enough people, you get a cloud of points that looks like a football at an angle - sloped, mostly points in the middle, and the middle points are further away from the regression axis than either end.

The instances you're interested in are those in the middle of the graph, far off the axis. And they do exist, but, statistically are balanced out by the ones on the other side of the axis. I'm sure there's more math to be done, but that's where my expertise ends.


GeriatricHydralisk t1_jaqvewx wrote

They have two special muscles. One is a sheet of muscle between the lungs and rest of the guts, beneath the lungs and attached to the shell, which squeezes the lungs to force gas out. The other is a sheet of muscle covering the rear opening of the shell (where the hindlimbs stick out). It's concave when relaxed, but flattens out when active, pulling the viscera back and expanding the body cavity. Since the guts don't change volume much on that short of a timescale (seconds), this forces lung expansion and inhalation.

Check the first two paragraphs and first figure of this paper:


GeriatricHydralisk t1_jamnit3 wrote

> unique scents that they can recognize

I swear, humans' poor sense of smell sometimes makes us like a blind person studying art, keenly aware of sculpture by hopeless at understanding paintings.

Imagine how much more we'd know about the natural world if we weren't almost "blind" to one of the most important senses of most animals.


GeriatricHydralisk t1_j9uj5yb wrote

>Also, over half of the species are aquatic or semiaquatic.

Yeah, I could see the original sentence being "148 species of terrestrial mammals", especially for a book generally focused on human history. Or "terrestrial" accidentally got lost in revisions and drafts and proofs etc.


GeriatricHydralisk t1_j9p5py2 wrote

It's worth noting, in addition to u/Jason-_B's excellent comment, that the placenta is not unique to mammals - it's seen in fish, lizards, and snakes as well. More importantly, unlike mammals, the intermediate states are still around, and plentifully represented.

In species with internal fertilization, the egg has to spend at least some time in the female regardless, just to add yolk and a shell. But more time in the female also lets her more precisely control the egg's environment, especially temperature, so keeping them interally has advantages (as well as the disadvantage of not being able to ditch them to escape a predator, and being "weighed down"). So a lot of species have variable time before laying, all the way up to laying right before hatching. Oxygen, CO2 and water can transfer, but it helps to ditch the shell in that case. However, no nutrient transfer occurs. At the very highest extreme, this is ovoviviparity - where the eggs entirely lack calcified shells, and the mom "lays" them immediately before or as the offspring are "hatching". From an outside perspective, this looks just like viviparity, but the key is the lack of nutrients - they need a yolk.

But if you've got eggs interally for a while, why not transfer some nutreints? There are lots of ways to do this, with the most bizarre probably being some species of caecilians (long, worm-like, burrowing amphibians) in which the mother grows nutritive lining in her uterus, which the young scrape from the walls and eat. However, a common way is to vascularize the yolk sac, squish it up against the uterus, let them fuse, and transfer nutrients across - bingo, you've got a placenta. Some of these are every bit as complex and specialized as mammal placentas.

The most useful thing is we have numerous independent evolutions of the placenta outside of mammals (who only evolved it once, as far as we know), as well as living examples of every intermediate you could ask for. There are even species (three-toed skinks) where some populations give live birth and others lay eggs.

Even crazier is the exception - Archosaurs (crocodiles, birds, dinosaurs, and their relatives) cannot ever evolve live birth. Unlike other species, the Archosaur embryo uses the calcium in the eggshell for bone calcification and, if the shell is removed, the hatchling is basically a gummy-bird or gummy-gator (obviously non-viable). This means they can never ditch the shell, and never take those first steps. And not a single Archosaur has ever evolved live birth, despite hundreds of millions of years of opportunities, and literally ruling the planet for most of that time.


GeriatricHydralisk t1_j9jklgp wrote

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.


GeriatricHydralisk t1_j9jfnpm wrote

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.


GeriatricHydralisk t1_j9jejqa wrote

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.


GeriatricHydralisk t1_j9gpv7o wrote

Rather than cover what's already covered elsewhere, it's worth pointing out why the AAH fails:

  • First, it considered traits "piecemeal", rather than looking at the organism as an integrated whole. This allows it to engage in the common fallacy of "remembering the hits and forgetting the misses" - it points to things like the diving reflex or subcutaneous fat that are consistent with diving, but "conveniently" ignores traits completely inconsistent with aquatic life, such as lack of reflexive swimming (babies show a diving reflex, but cannot actively swim) or valvular nostrils.
  • Second, it's completely at odds with comparative data. Lots of mammals have become semiaquatic and aquatic, and none of them have done so in the manner postulated by AAH. Nostril valves and webbed digits are near-universal in semiaquatic mammals, but absent in us, nothing else has become bipedal to move in water like AAH proposes. There are even several monkeys which swim and dive on a VERY regular basis (Allen's Swamp Monkey, Japanese Macaque, Proboscis Monkey), and a) don't display anywhere near the strength of adaptations claimed by AAH and b) have the sort of adaptations you would expect from a typical swimming/diving mammal.
  • Lastly, back when AAH was proposed, and when all the major books/articles/talks in favor of it came out, we knew almost nothing about our ancestors, particularly their habitats and ecology. The Leakeys had only just begun their work, and wouldn't find Lucy until the 70's.

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.


GeriatricHydralisk t1_j8j41wp wrote

Like everything in biology, there's one, unified answer: It depends.

Specifically, how do you define "exoskeleton"?

If you define it broadly enough to simply be a hard, outer layer, then basically everything with armor fits the bill: turtles, armadillos, oysters, etc. Even a variety of protists like diatoms.

If you say it has to support body weight, it gets more tricky, but there a few examples that squeeze in here: turtles again, crocodiles have specialized muscles to "brace" against their armored hide when walking, etc.

If you specify that it has to be involved in turning muscle shortening into body motion, like in arthropods and with our endoskeleton, nematodes may fit the bill, as they have a collagenous "cuticle" which inertnal muscles attach to. But it's not truly "rigid" the way an arthropod exoskeleton is (think of it like stiff rubber).


GeriatricHydralisk t1_j7p82qz wrote

Tendons heal and remodel slowly simply because they're mostly collagen with very few cells, compared to muscle (almost all cells) and bone (which has a surprisingly high number of cells). Even with every cell working flat-out to fix/alter the tendon, there's just soooo much collagen and so few cells that it takes forever.