I occasionally get questions like this one: “What do you consider the most convincing evidence for evolution?” My answer is usually “the fossil record combined with dating methods,” but I often add that “the evidence from biogeography is so convincing that I’ve never seen a creationist even try to rebut it.” (You can see some of the biogeographic evidence in chapter 4 of Why Evolution is True, and I give the fossil evidence in Chapter 2.)
And if someone asks me, “What’s the most convincing evidence for human evolution?”, I’d also give the first answer above. That’s because the temporally ordered record of human evolution shows a fairly clear progression from the morphology of an ape somewhat like a chimp (i.e., our common ancestor with the chimp and bonobo that lived about 6.4 million years ago). It’s not a straight line pathway, and we don’t know all the details, for human evolution, like all evolution, is a branching bush, and some branches went extinct.
When I was on a BBC Three show, “Conspiracy Road Trip,” with each of us assigned to convince a group of British creationists of the truth of one bit of evolution (mine was to dispel Noah’s Ark and the great flood scenario), the most convincing evidence to the creationists was the presentation of an evolutionary series of hominin skulls by Tim at White at Berkeley. That bit begins at 42:26 in the video below (I appear earlier).
This week I got a note from an upset parent whose child attended a religious school where the kid was told that humans could not possibly have descended from apes. I responded that humans were apes, and we descended from a common ancestor with chimps (and from other ancestors with other primates)—an ancestor that, I suspect, looked rather chimplike. (It is of course a misconception that we descended from living chimps.)
I tried to help the parent by giving him evidence for human evolution, and that included this photo from the Smithsonian, posted on Talk Origins, Doug Theobald’s site), showing (with the exception of the skull at the top left corner), various hominin skulls laid out in temporal order.
Figure 1.4.4. Fossil hominid skulls. Some of the figures have been modified for ease of comparison (only left-right mirroring or removal of a jawbone). (Images © 2000 Smithsonian Institution.)
- (A) Pan troglodytes, chimpanzee, modern
- (B) Australopithecus africanus, STS 5, 2.6 My
- (C) Australopithecus africanus, STS 71, 2.5 My
- (D) Homo habilis, KNM-ER 1813, 1.9 My
- (E) Homo habilis, OH24, 1.8 My
- (F) Homo rudolfensis, KNM-ER 1470, 1.8 My
- (G) Homo erectus, Dmanisi cranium D2700, 1.75 My
- (H) Homo ergaster (early H. erectus), KNM-ER 3733, 1.75 My
- (I) Homo heidelbergensis, “Rhodesia man,” 300,000 – 125,000 y
- (J) Homo sapiens neanderthalensis, La Ferrassie 1, 70,000 y
- (K) Homo sapiens neanderthalensis, La Chappelle-aux-Saints, 60,000 y
- (L) Homo sapiens neanderthalensis, Le Moustier, 45,000 y
- (M) Homo sapiens sapiens, Cro-Magnon I, 30,000 y
- (N) Homo sapiens sapiens, modern
Note that the skull at upper left is the skull of a modern chimp, so it doesn’t really belong with the others. It’s just there for comparison. But look how things change over time: the face gets pulled back, the teeth get smaller, the brow ridges shrink, and most evident, the braincase gets larger.
Creationists have big trouble with this because they don’t know where to draw the line between “apes” and “humans”. Some maintain that every fossil earlier than some arbitrary one (say a Homo habilis) is an “ape”, while everything after that is simply a human (they might even say “a malformed human”!) But that tactic is so arbitrary and capricious that it’s not convincing even to some of the British creationists above.
I like the photo simply because it’s a wonderful piece of evidence for human evolution, with the skulls laid out in temporal order. (Now they’ve eliminated the “robust” hominins, and that would confuse things a bit though it would be more accurate, for the robust hominins are still hominins. It also leaves out more recently discovered fossils such as Homo floresiensis, the tiny “hobbit” hominin that went extinct about 50,000 years ago.
Also, we don’t know that this is the line of evolution to modern humans (and it probably isn’t), but it does show gradual change over time that’s undoubtedly genetic, and that is what evolution means. We do not see fossils resembling modern humans 3 million years ago, but we see them now. The earliest hominin skulls we see resemble the skulls of early apes, and gradually evolve into skulls that look like those those of modern humans. What better evidence of human evolution could we wish for? I’m always amazed that fossils really exist, and also that human fossils are especially rare—yet there are enough of them to provide convincing evidence that our species evolved from a common ancestor with other apes.
Putting the chimp skull in the figure does cause some confusion, as described at Anthropology.net by Kambiz Kamrani:
I have some slight problems with this image, though. The biggest problem, and a common misconception I see in regards to understanding human evolution, is the whole we descended from chimpanzees train of thought. This image compounds it. The lineage of primates that have become the chimpanzees have been evolving independently of the human lineage. And because the non-human primate fossil record is rather spotty — it is hard to see these types of trends and transitions that we see in the above image happen along in chimpanzees.
Working on that note, this composition implies that our ancestral form was a chimp and once the chimp and human lines diverged then humans went through many natural selection events while chimps just remained stagnant as chimps. That’s wrong. Chimps and humans share a common ape ancestor.
But if you point out that the modern chimp skull is simply there for comparison, and that in all likelihood is fairly similar to the skull of our common ancestor with modern chimps, the problem disappears. Still, many people think that we evolved from modern chimps, and it takes some doing to dispel that idea by explaining the branching pattern of evolution and the idea of common ancestry. Those are a bit harder.