Why did humans take so long to evolve?

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One of the enduring mysteries of the evolution of humans is why it took so long for us to evolve: more than 60 million years after the origin of primates.

Primates (https://en.wikipedia.org/wiki/Primate) arose early among mammals. In most ways, their anatomy remains primitive. This includes Hominidae (https://en.wikipedia.org/wiki/Hominidae).

Humans have specialised hind feet (https://en.wikipedia.org/wiki/Foot). However, in other respects we remain primitive and unspecialised relative to other mammals, in anatomical terms.

Therefore, it is puzzling that humans did not evolve already in the Miocene (https://en.wikipedia.org/wiki/Miocene) or Pliocene (https://en.wikipedia.org/wiki/Pliocene).

The unspecialised, primitive anatomy of humans can best be seen relative to cetaceans. The latter are obviously extremely modified relative to the ancestral mammals that arose in the time of dinosaurs (https://www.pbs.org/wgbh/nova/sciencenow/0303/02-mya-nf.html#:~:text=Dryomomys%20is%20the%20most%20primitive,the%20pen%2Dtailed%20tree%20shrew.).

Furthermore, even the anatomy of the brain is more modified in cetaceans than it is in humans (https://medium.com/the-vagus/the-brain-behind-the-bottlenose-dolphin-3c2ff3f30ff6 and https://a-z-animals.com/blog/dolphin-brain-vs-human-brain-what-are-the-differences/ and https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0257803#:~:text=Cetaceans%20such%20as%20the%20sperm,Homo%20%5B3%2C%204%5D. and https://academic.oup.com/biolinnean/article/133/4/990/6263583?login=false).

On land, various important lineages of mammals, such as chiropterans (https://en.wikipedia.org/wiki/Bat), perissodactyls (https://en.wikipedia.org/wiki/Odd-toed_ungulate), and proboscideans (https://en.wikipedia.org/wiki/Proboscidea), are incomparably more modified anatomically than are primates, including humans.

The human pes (https://en.wikipedia.org/wiki/Pes_(anatomy)), although unique among mammals, is not as modified as the manus (https://en.wikipedia.org/wiki/Manus_(anatomy)) of bats (https://en.wikipedia.org/wiki/Bat_wing_development).

The feet of equids (https://en.wikipedia.org/wiki/Equidae), which are unguligrade (https://en.wikipedia.org/wiki/Ungulate) plus reduced to one remaining claw per foot, are far more modified anatomically than any part of the human body.

The proboscis (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1749-4877.2012.00315.x and https://www.researchgate.net/publication/236183185_Structural_and_functional_comparison_of_the_proboscis_between_tapirs_and_other_extant_and_extinct_vertebrates) and dentition of proboscideans are far more modified anatomically than any part of the human body.

And yet cetaceans (including forms with braininess similar to that in humans, https://en.wikipedia.org/wiki/Cetacean_intelligence and https://www.karger.com/Article/Fulltext/454797 and https://www.scientificamerican.com/article/cetaceans-rsquo-big-brains-are-linked-to-their-rich-social-life/ and https://blogs.scientificamerican.com/news-blog/are-whales-smarter-than-we-are/ and https://australian.museum/blog/amri-news/fossil-evidence-sheds-light-on-why-whales-and-dolphins-have-large-brains/), chiropterans, equids, and elephant-like forms evolved before hominids did.

The human hand, and the opposability of its digits, are functionally remarkable, as manifestations of the unrivalled technical ability of humans.

However, the manus itself is hardly different, anatomically, from those of the first amphibians to crawl out of the primeval slime. The advances have been neurological rather than anatomical, i.e. matters of software rather than hardware.

Furthermore, many kinds of amphibians have some dexterity (https://en.wikipedia.org/wiki/Fine_motor_skill). Some show opposability (https://www.collinsdictionary.com/dictionary/english/opposable) comparable to that seen in hominids.

It is easy to assume that there was some special contingency in Pliocene Africa, without which hominid-like primates could not have begun to arise (https://en.wikipedia.org/wiki/Human_evolution).

However, American monkeys (https://en.wikipedia.org/wiki/New_World_monkey) seem in various ways (apart from their currently limited body size) to be even better-suited to 'human' ancestry than apes are.

Many of the American monkeys have heads reminiscent of miniature humans (http://news.bbc.co.uk/cbbcnews/hi/newsid_5390000/newsid_5395300/5395300.stm and https://www.marmosetcare.stir.ac.uk/understanding-behaviour/faces.html and https://www.dreamstime.com/stock-photo-white-faced-capuchin-monkey-cebus-capucinus-close-up-face-zoo-panama-image78812984 and https://www.shutterstock.com/image-photo/close-face-white-faced-capuchin-monkey-497041303 and https://living-links.org/the-primates/capuchin-monkeys/).

Cebidae (https://en.wikipedia.org/wiki/Capuchin_monkey) are brainier than most apes, with a capacity for tool-use.

Furthermore, the anatomical modifications for brachiation (https://en.wikipedia.org/wiki/Brachiation), in certain American monkeys, would seem somewhat preadaptive for bipedality.

Humans are unrivalled, in the animal world, in our ability to invent and apply tools (including weapons) and artificial constructions.

There are at least three levels of adaptation involved in the evolutionary progression of animals, viz.

  • anatomical,
  • cognitive (depending on the size of the brain, rather than its anatomy), and
  • cultural (particularly facilitating social spreading of technical innovations).

Humans are anatomically unadvanced, except somewhat in the case of the foot. However, our cognitive capacity is advanced (albeit not necessarily more than that of certain cetaceans). And our cultural advancement is so far beyond that of other animals that we are in a category of our own.

The basic evolutionary innovation of humans is technical development, beginning with the manipulation of fire for cooking.

Opportunities to benefit from a cultural-technical strategy have presumably existed ever since any mammal evolved to a certain level of braininess.

The main advances of human evolution have been matters of neurological software and cultural versatility. Therefore, it would seem possible for these advances to have been made as soon as any primate lineage achieved a level of braininess seen in many clades of animals today, including

Given the evolutionary advantages conceivable in a wide range of biomes at various times in the last 50 million years, why could these cultural-technical advances not have been produced by natural selection already, say, 30 million years ago - instead of beginning only five million years ago with the appearance of Australopithecus (https://en.wikipedia.org/wiki/Australopithecus)?

Posted on August 07, 2022 01:40 AM by milewski milewski


Perissodactyla arose about 55 million years ago (https://research.amnh.org/paleontology/perissodactyl/evolution/intro#:~:text=The%20mammals%20that%20lived%20in,the%20beginning%20of%20the%20Eocene.). The most specialised modern genus, Equus, arose about five million years ago (https://en.wikipedia.org/wiki/Evolution_of_the_horse).

Posted by milewski almost 2 years ago

Interesting article. Thanks. Could American monkeys evolve into something more human, with time, and if not, why not ?

Posted by botswanabugs almost 2 years ago


Now that humans have invaded the Americas, the niche for such animals is filled, presumably precluding any further evolution of monkeys in the cultural-technical direction. However, it is puzzling that American monkeys did not evolve this way in the past, producing - by way of evolutionary convergence with Australopithecus in Africa - a kind of American 'parahominid' lineage.

Please bear in mind that one of the enduring mysteries of human evolution is why Homo took so long to reach the Americas. Homo colonised Eurasia millions of years ago, and colonised Australia more than 50 thousand years ago despite that continent never having a land-bridge from Asia, even at lowest sea levels. The Americas lay empty of any primate resembling hominids, until perhaps 20 thousand years ago, when Homo sapiens finally began to colonise them.

What this means is that there has, for perhaps up to ten million years, been an 'empty niche' for a cultural-technical monkey-descendent, with upright bipedality and advanced dexterity, in the Americas.

Primates were present in South America already 36 million years ago, and these evolved into monkeys starting about 18 million years ago (https://en.wikipedia.org/wiki/Great_American_Interchange). So, a clade potentially capable of producing first australopithecus-like, and then Homo-like, descendents has been in place in South America for perhaps ten million years.

Instead, in reality, this opportunity was left unrealised.

In the end, the niche for a human-like animal in South America was filled not by in-situ evolution, but instead by intercontinental recruitment - and so late that, by then, Homo neanderthalensis had already come and gone in Europe. The neanderthal species was already extinct by about 40 thousand years ago (https://www.bbc.com/news/science-environment-60305218), 20 thousand years before Homo ever set foot on the Americas.

Posted by milewski almost 2 years ago

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