This is not intended to be a loaded question. I absolutely believe in evolution, and do not question that. I will be honest, I was having a conversation with a friend about the X-men, and it made me realize I don’t actually know what separates the stages of human evolution. A cursory google search says a lot about behavior, but I feel like there must be something about physical involved, as well. So what are the criteria for something to be one stage of human evolution, versus another?
In: Biology
Physical differences really. Things like teeth, size of the brainpan, shape of the skull, posture.
Homo habilis is an ancestor of ours that lived around 2.6 million years ago. He was shorter, with much longer arms. He had a much less protruded face which made him look less like an ape and more like a human. Its cranial capacity was less than half of ours and its teeth still had big canines.
So while it’s definitely an ancestor species of ours. It’s not hard to picture Homo Habilis being decidedly different.
If you take a big leap closer, you have for instance Homo Neanderthalensis and Homo Erectus.
Homo erectus was the first hominid to really start looking like us. They walked upright with flat faces and prominent noses. Their brain size varied wildly across populations and it seems like they grew up much faster than us. Which interestingly made them very human with a lot of tool use yet at the same time very different lifestyles.
Neanderthals co-existed with us and were closely related enough that we could interbreed. Neanderthal genes in our gene pool likely helped us adapt and spread across the world. But they were clearly a different species. Shorter than us but more heavily set. They had bigger brains, heavy brows and weak chins. You wouldn’t have a hard time telling neanderthals apart from homo sapiens.
The interesting thing is that we can tell a lot of human species apart. But it’s a lot more difficult to discern which are our direct ancestors. And which are human species that co-existed with us much like chimps, gorilla’s and orangutangs exist at the same time in the world today.
It’s not a clear cut line of one hominid species evolving into the next. Different homo species seem to have lived alongside each other and even interbred with each other at times. Our neanderthal genes seem to have played an important role in our success.
Evolution doesn’t proceed in stages – that’s just academic categorization that we impose on nature when we classify and study things. Breaking things into stages helps us organize and research but you have to be careful not to let the artificial stages be anything more meaningful than somewhat arbitrary lines we draw.
Evolution proceeds in fits and starts, with “branches” shooting off all the time. Most of those branches wither and die out immediately, a few last a little bit and when one is successful, a new species takes root. But even then, there’s no real bright lines. Nature and evolution are hazy, more like a shrub tangled up with vines than a solitary tree.
All that said, we tend to characterize hominid species by cranial cavity size (which presumably relates to actual brain size), skull shape, dentition, how upright they stood, etc. But there’s no real linearity to human evolution – our ancestors didn’t follow a straight path from one hominid species to the next to the next and so on. Rather, for most of hominid evolution, there were likely (very likely) various species of hominid on the Earth at the same time, some capable of interbreeding successfully. It is only in the last 300 centuries or so that one hominid species came to dominate and wipe out all others.
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