Dear Straight Dope:
How certain is it that all humans evolved from one common primate ancestor? Is it possible that the all the organisms on this planet that we now lump together as humans could actually be the roughly identical results of several different branches of primate evolution?
SDStaff DavidB replies:
It’s pretty certain, assuming by “one common primate ancestor” you don’t actually mean a single individual. A single group or species, yes.
While evolution can produce similar results among differing species (for example, birds, insects, and bats, all of which evolved wings independently), it doesn’t converge two different species into one. To put it more precisely, two species can’t merge so completely that they appear, even through DNA tests, to be a single species.
The DNA tests are key. In all the tests performed on people since DNA testing started, nobody has ever noticed that some results were completely different from others to the point of being a separate species. It’s considered highly unlikely, to say the least.
Fossil evidence also stands against the idea. I can’t create an evolutionary tree diagram very easily here, but there’s a good one in the January 2000 issue of Scientific American (“Once We Were Not Alone,” by Ian Tattersall, p. 60). The article talks about how there used to be a number of different hominid species sharing the planet, but now we’re down to just us. The accompanying diagram shows the evolutionary tree branching a bit differently from when I went to school and it was thought that A begat B begat C in a nice linear arrangement. Not so, say scientists now. Instead, you see the classic branching of most other species: Australopithicus anamensis begat Australopithicus bahrelghazali and Australopithicus afarensis. A. afarensis begat A. africanus, A. garhi, and Paranthropus aethiopicus. P. aethiopicus begat P. robustus and P. boisei. Somebody in the Australopithicus line begat Homo rudolfensis and Homo habilus, who begat Homo ergaster. H. ergaster begat H. erectus and H. antecessor, who begat H. heidelbergensis who begat H. neanderthalis and good old Homo sapiens (us).
Throughout all of the begetting, various species shared the land with similar hominids. But in the end, only Homo sapiens survived, though we shared the world with Neanderthals until as little as 30,000 or so years ago. While we don’t know for certain how they interacted, the authors of the Scientific American article note, “In light of the Neanderthals’ rapid disappearance and of the appalling subsequent record of H. sapiens, we can reasonably surmise that such interactions were rarely happy for the former.”
The authors go on to note, in a more direct answer to your question, “the repeated pattern at archaeological sites is one of short-term replacement, and there is no convincing biological evidence of any intermixing . . .”
In 1982 Cecil wrote a column that dealt tangentially with the claim that the different races actually evolved separately from different species (http://www.straightdope.com/classics/ a1_099.html). He described an alternative hypothesis about human evolution that was then being discussed: “To put it another way, Caucasians are most ‘advanced,’ Mongoloids slightly less so, and Negroids least of all. The most elaborate expression of this theory was given by the anthropologist Carleton Coon in the mid-60s. Coon’s idea was that there originally were five basic races that evolved separately, in widely differing times and places, from our Homo erectus forebears — Caucasians, predictably, being the first.” Cecil went on to state that Coon was not a crackpot, and there was some fossil evidence supporting this view. But, he said, “there are some major objections to it as well, the most obvious being that one would expect races that had evolved separately to be unable to interbreed, as all humans today clearly can.”
That was then, this is now. Now we have genetic techniques that weren’t available back when Cecil wrote that column. Those techniques show that Coon’s ideas don’t pan out.
Coon himself died in 1981, but his son, Carleton Jr., has tried to incorporate this new information into his father’s hypothesis. Carl Jr. (the man I’m discussing here, not the restaurant chain) has a website dedicated to “Progressive Humanism” (http://www.progressivehumanism.com/) and also has a book coming out in June (Culture Wars and the Global Village, Prometheus Books). In both he puts forth some of his views on human evolution. From a chapter of the upcoming book, which he e-mailed to me, the hypothesis he favors is that “Homo sapiens evolved relatively recently, in Africa, and spread out, as the geneticists think they have demonstrated. But as modern people dispersed, they interbred with the local populations of erectus. At least some of the offspring of these unions were ‘sapient’ in their possession of large brains that could conceptualize, and a capacity for languages that gave them the verbal tools they needed for what now passes as human thought. Such hybrids were doubly advantaged in terms of their chances of surviving and passing on their genes in the specific environment in which they were born. They were the new models of the old winners, according to this explanation, and became the ancestors of modern racial groups.” Basically, he’s saying that Homo erectus was already adapted into “races” (much like his father said) and members of Homo sapiens, by interbreeding with erectus, adopted existing racial characteristics rather than evolving them on their own.
I suppose it’s not impossible, but as he even notes on his web page: “Majority opinion favors the so-called ‘radiation’ theory, based on the genetic evidence.” In other words, most people in the field think Homo sapiens beat out everybody else and evolved on its own, in the process splitting into the superficially different varieties we call races.
That said, even if Coon is 100% correct, it still means the answer to your question here is “no.” Even if there was some interbreeding between racially subdivided Homo erectus populations and Homo sapiens populations in the ancient past that led to racial differences, we’re all still one species. I only bring it up as an interesting side note and because Cecil mentioned the idea in that previous column, and I know that someone, somewhere, would have asked about him if I didn’t.
So, I’m afraid it’s just Homo sapiens from here on out. We won the race and beat our cousins into extinction, either metaphorically or literally.
Send questions to Cecil via firstname.lastname@example.org.
STAFF REPORTS ARE WRITTEN BY THE STRAIGHT DOPE SCIENCE ADVISORY BOARD, CECIL'S ONLINE AUXILIARY. THOUGH THE SDSAB DOES ITS BEST, THESE COLUMNS ARE EDITED BY ED ZOTTI, NOT CECIL, SO ACCURACYWISE YOU'D BETTER KEEP YOUR FINGERS CROSSED.