One of the more common claims among creationists is that while microevolution has been observed there is no evidence for macroevolution. I think I have more than adequately refuted that claim with my blogs showing some of the evidence of chimp/human common ancestry. However, I thought it might be interesting to take an in-depth look at what the fossils evidence of human origins really is.
In order to understand the depth of the evidence one needs to understand some basic information first. This first post will be devoted to providing some of the more important background information.
Genetic evidence clearly shows humans to share a relative recent common ancestry with the great apes: chimps, gorillas, and orangutans. In fact, chimps and humans are more closely related genetically than are either to gorillas or orangutans. Yet on a superficial level, chimps seem to share more traits with gorillas and orangutans than they do with humans.
What does that mean? Well, for one thing it suggests that humans have been evolving more rapidly than chimps ... and that means that the chimp more closely resemble our common ancestor than do humans. But what could that common ancestor have been like? How are we different, and how are we alike? To get an idea of what the answer should be let's compare chimp and human skulls.
This is a picture showing the two side by side. In case anyone is wondering the chimp is to the left. Notice the differences:
(1) The face of the human skull is significantly flatter. This is called ORTHOGNATHUS. Species that have their jaws protruding in front of their face are called PROGNATHUS. The importance of this is that in humans there isn't as much room for teeth to be situated in the jaws. Yet, humans and chimps (indeed, all the ape-like primates) share the same dental formula, 2.1.2.3 both in the upper and lower jaw. That means normal adults have in each side of both the upper and lower jaws two incisors, 1 canine, 2 premolars, and 3 molars. Or in other words, 16 teeth in the upper jaw and 16 teeth in the lower jaw.
If humans have less jaw room then how do they get all those teeth in there? The answer is "poorly". One thing we do is have smaller teeth, but not all that much smaller. This leads to some interesting observations. Any dog owner can tell you that dogs have a large dagger-like tooth. These teeth are in fact named after dogs ... the canine teeth. In the picture above you can see that chimps too have pronounced canine teeth. It is this tooth that is most reduced in humans compared to chimps.
A feature of animals that are fully jawed, the prognathous ones, is that on each side of the mouth the back teeth (molars and premolars) run parallel. In orthognathus humans we don't get that parallel arrangement. Instead the teeth are arranged in a smooth curving U shape.
Dogs and their near relatives, wolves, use the canine teeth to capture prey. But for the most part chimps are frugivores (fruit eaters). So what do chimps use them for? Well for one thing, the work of Jane Goodall has shown us that chimps are not ONLY frugivores, they do hunt monkeys and eat meat. Thus, they could benefit from having those teeth for that purpose. However, chimps also get into fights with other chimps. One way a chimp protects itself in fight is to bite. So chimps benefit from the large canines there as well.
But note ... canine teeth are often long and they are used to bite living, struggling animals. Because of this these teeth are subject to severe stresses. They need to be well-rooted inside the skull or else they can be easily broken or pulled out. Fortunately for the chimp they are well-rooted. Look at the picture above. You can see in the skull a pronounced ridge extending from the canine tooth in the upper jaw to a little past the side of the nose. The root of the canine tooth lies in a cavity below that ridge.
But what about humans? The human canine does not extend much beyond the other teeth. We do not normally bite living, struggling animals so our canines are not subject to the same stresses as are the chimps. Yet our canines are rooted almost as deeply as are the chimps. Look at the picture of the human skull (or even better yet, feel the ridge on your own upper jaw). The rooting also proceeds a little higher than the base of the nose. This is more stabilization than we really need and the fact that we have it is one piece of evidence that we do share common ancestry with chimps.
Another way we cope with having less room is by delaying the date that all of our teeth erupt into the jaw. In humans our final molars do not normally appear until sometime between age 17 and 25. These are called our wisdom teeth. Furthermore, there often isn't enough room for them to erupt normally. That is why wisdom teeth are so often pulled. If you look in the picture of the human skull above you can see the wisdom tooth in the upper jaw growing in at a strange angle. The wisdom tooth has been pulled in the lower jaw.
If you think that wisdom teeth are not a particularly adaptive idea, you are not alone. The only reason I can think of for it to be there is one of historical contingency. That is, we evolved from ancestors with three molars, just like chimpanzees happen to have.
(2) Other important features of the skulls includes the crest that goes front to back on the chimp skull It is called the SAGITTAL CREST. It forms a site for jaw muscles to attach. Consequently chimps have much stronger jaw muscles than we do ... all the more effective for an organism that defends itself by biting.
Also if you look carefully you can see another crest that runs transversely on the back of the chimp skull. That is the NUCHAL CREST. Humans have their head over the spinal column. Therefore, the spinal column can help support the skull against the force of gravity. Chimps, on the other hand, hold their heads in front of the spinal column. Their heads must be supported by neck muscles. Thus, a chimp's neck muscles need to be stronger than ours. The nuchal crest serves as a site of attachment for these strong neck muscles.
(3) Notice the prominent brow ridges in the chimp skull. Humans do not have that. The frontal lobes of our brain are so large that our skull takes up that space such that there is a smooth surface above our eyes. This is called FRONTAL BOSSING.
This a picture of the bottom side of the skull. One thing that I want you to notice is the position of the hole in the skull that the spinal cord exits. That hole is called the FORAMEN MAGNUM. Notice that in humans it is much more on the bottom surface of the skull. In chimps it is coming out the back. The reason for this is that humans normally walk upright on two feet, while chimps are knuckle walkers. That is, chimps normally use their arms for walking. Because of that chimp's heads and spine are horizontal instead of vertical like ours. Having the foramen magnum in the back makes such a posture easier for chimps while having it on the undersurface of the skull makes it easier for us. This is an important adaptation. So important, in fact, that paleontologists use the location of the foramen magnum as a major piece of evidence to determine whether or not a particular fossil species walked upright (like us and therefore likely to be on the evolutionary line that leads to us) or on all fours (like chimps and NOT likely to be on the evolutionary line that leads to us).
In future posts I will look at many of the fossil skulls (since these have the most details). These details of the skull along with the brain size within the skull will be major pieces of evidence that we need to consider.
So until then ... Cheers,
Darwin's Beagle
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That was fascinating. Thanks.
I shall look forward to future posts in this series.
/jkh