Evolution 101

Saturday, May 20, 2006

How Did Humans Evolve?

You’ve no doubt noticed the opening music that I’ve added. It’s the “Sunrise” piece by Richard Strauss, of the work, “Thus Sprach Zarathustra.” It’s more popularly known as the theme to the movie, “2001: A Space Odyssey,” and it’s from that usage that I take my inspiration. The opening scenes of the movie are subtitled, “The Evolution of Man,” and show a group of ape-like creatures learning how to use tools, and thus, become human. This aspect of evolutionary theory- the treatment of humans by evolution- is one of the central interests of the theory, because let’s face it- we humans are continuously preoccupied with ourselves. You’ve also seen this in the logo for the podcast- a variation of the classic, “March of Progress” imagery that shows hominids walking in a line at side profile, beginning with an ape-like creature and ending with a modern human.

So I thought that this week I would talk about the evolution of humans. I think we’ve tackled enough relevant topics so far to begin investigating the subject. The molecular evidence showed very clearly that chimpanzees are our closest living relatives, and so, logically, we must share an ancestor in common at some point in the past. This common ancestor, or concestor, wasn’t necessarily identical to modern chimpanzees- remember, all populations are in a state of evolutionary flux, it’s just that some are required by their environments to evolve faster than others. So although the human-chimpanzee concestor wasn’t a chimpanzee, we’d probably recognize it as being more chimpanzee than human if it were alive today.

So, humans and chimpanzees are both descended from an ape-like concestor. When did the lines split into human-only and chimpanzee-only lines? The answer may not be as cut and dry as you might think. The best theories based on the fossil evidence indicate that our concestor lived between 5 to 7 million years ago, at which point evolutionary forces caused one population to evolve human-like characteristics while the other line evolved more chimpanzee-like characteristics. However, new evidence has just been made available that shows by examining the human and chimpanzee genomes that human and chimpanzee ancestors diverged and then converged, before diverging for a final time less than 5 million years ago. Genetic analysis suggests that humans and chimpanzees evolved into separate species which then interbred, forming a hybrid species which then bred back into one of the parent populations. It’s not clear whether this human-chimpanzee hybrid returned to the human or the chimpanzee population, but the molecular evidence is clear that the hybridization did happen- the X chromosome has a particularly recent connection to the chimpanzee genome. This means that human-chimpanzee hybrid males would have been infertile, but the females were not, and thus returned back to the parental population, mixing chimpanzee and human genes each time. This new study by the Broad Institute in Massachusetts is scheduled to be published in Nature later this year, but the results have been made available on the Internet, so I’m sharing the scientific cutting edge with all of you.

But regardless of the human-chimpanzee hybrids, eventually the two lines did split for good. And gradually, our ancestors changed from being something that was willing to mate with a chimpanzee, into something that would rather hunt them for food, train them for entertainment, or sequence their DNA. What was the first step? The first step, as it seems, is literally a step. A bipedal step, to be precise- the first thing to distinguish our ancestors from chimpanzee ancestors is the ability to walk upright. But being able to walk upright doesn’t earn the scientific, phylogenetic designation of human- we designate all human species by the genus “Homo” as in our binomial, “Homo sapiens.” But these first human ancestors weren’t human enough to be considered part of our genus, and instead are called, “Australopithecus.” One species of this genus in particular is thought to have been ancestral to humans- Australopithecus afarensis, one specimen of which has been nicknamed, “Lucy.” Like most of the Australopithecines, Lucy lived in Africa.
Lucy, and the rest of her species, resembled chimpanzees in a lot of ways, but one difference is obvious- she walked upright, like a human. And not just sometimes, the bone structure of her pelvis indicates that she was upright most of the time.

The next big change in human evolution was the expansion of the brain. This was different than a lot of scientists had expected- they had assumed that a larger brain would have been the first change in the human-chimpanzee divergence, followed by other human traits such as bipedalism and tool use. This turned out not to be the case- walking upright evolved first. But the expanding brain followed soon after, and in fact it’s how we classify human species- that is, species that belong to the genus “Homo.” The first human, or at least the first recognizable human species to which we’re willing to give the designation, is the Handyman, Homo habilis. The Handyman lived between 1.5 and 2.5 million years ago, and he gets his name because rudimentary tools have been found with fossils of this species. These tools weren’t anything spectacular- just flakes of stone used as rudimentary knives, for the cutting of meat off dead animals. It’s unlikely that the Handyman was a hunter- more likely, he would have taken meat from already dead animals like a scavenger.

After Homo habilis, we find the next major step in human evolution. Homo erectus, or the Upright Man arose in Africa about 1.5 to 1.8 million years ago. Homo erectus had a larger brain than Homo habilis, and its anatomy was more similar to modern humans. But the most interesting thing about Homo erectus was its incredible success- it was the first human species to engage in actual hunting, and this had the effect of expanding its territory. Because its diet became more reliant on animals than plants, Homo erectus began to migrate- and thus spread out of Africa, and colonized southeast Asia, even going up farther north into Eurasia. There is also evidence that Homo erectus was able to control fire. There is some controversy about whether Homo erectus evolved into a separate species once it migrated out of Africa and into Asia, but even if this happened, the two species are so similar to make it almost impossible to tell today.

Homo erectus is the last major evolutionary transition before we get to modern humans, Homo sapiens. But how did this transition take place? There are a couple hypotheses- the “Out of Africa” hypothesis suggests that Homo sapiens evolved from the Homo erectus population back in Africa, and migrated out again, following the path that Homo erectus had taken earlier. The multiregional hypothesis suggests that Homo sapiens evolved in different geographical locations independently from different Homo erectus populations. This would suggest that European Homo sapiens evolved from a European population of Homo erectus, and the same is true of Asians, Africans, and Indonesians. This latter hypothesis is looking weaker and weaker as the genetic evidence piles up- any given human isn’t that significantly different from another, whatever the geographical origin. Richard Dawkins has come out in support of an “Out of Africa again and again” hypothesis, which suggests that Homo sapiens migrated out of and back into Africa several times before finally spreading out over all the continents. This hypothesis is backed up by genetic evidence tracing the genetic similarity of various genes among different human populations, and it looks the most promising. One of the major differences setting Homo sapiens aside from the other homonids is our use of language. This development is likely what allowed modern human society to expand and become as complex as it is now.

But what about the Neandethals? I haven’t forgotten them. Homo neandethalensis doesn’t figure in human ancestry- they aren’t direct ancestors. Analysis of mitochondrial DNA found in Neandethal fossils has confirmed this. What is most likely is that Neanderthals evolved from European populations of Homo erectus, and were either hunted or out-competed by the our ancestors, the Homo sapiens that had migrated into Europe from Africa. So you can think of them as our evolutionary cousins, if you like.

So that’s the basics of human evolution. The transitions aren’t really as simple as I’ve made them seem, and there are several subspecies that are transitional between the major species, but by and large, this is what you should know. After diverging with the other great apes, bipedalism evolved in the Australopithecines, but they weren’t human quite yet. Once a large enough brain evolved, rudimentary tools began to be used, as seen in Homo habilis, the Handyman. These then became migratory hunter/gatherers, as seen in Homo erectus. Modern humans evolved the use of language, and migrated out of Africa and all over the world, to where we are today.

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