Hominids and Hominid Transitional Forms

Eventually we had to talk about this, after all. Take a look at the class hypotheses. Some of you show humans and apes having a common ancestor; some of you don't. Look also at the geologic time page. One of the things you'll note is that apes appear earlier in the fossil record than humans do.

Based on this, if a hypothesis says that apes and humans have a common ancestor, what sort of fossils or sequence of fossils of ape-like and/or human-like organisms would it predict (Don't worry about specific anatomic traits here -- we'll get to that)? Explain.

Please write your answer/response on the separate sheet.

If a hypothesis says that apes and humans do NOT have a common ancestor, what sort of fossils of ape-like and/or human-like organisms would it predict? Explain.

Please write your answer/response on the separate sheet.

Apes are not monkeys. Monkeys are organisms in two major groups, the Old World monkeys (found in Africa and Asia) and the New World monkeys (found in South America). Old World monkeys have relatively short tails; New World monkeys have long tails which can be used to grasp objects, swing from branches, etc. Apes are tailless, larger than monkeys, and have larger brains than monkeys. The apes include chimpanzees (Pan troglodytes), bonobos or pygmy chimpanzees (Pan paniscus), Gorillas (Gorilla gorilla), and orangutans (Pongo pygmaeus). Some primatologists include gibbons (Hylobates spp.) as apes also, but most would separate the gibbons into a group of their own. Chimps, gorillas, and pygmy chimps are African. Orangutans are Asian.

Let's consider ways that apes and humans differ. First, humans are fully bipedal; that is, we walk on two legs. Apes can walk bipedally for short distances, but not very efficiently. How are these differences reflected in skeletal anatomy?

One difference is the position of the opening in the skull where the spine joins to it, and where the spinal cord nerves connect with the brain. This opening is called the foramen magnum, which is Latin for "big hole."

Foramen Magnum Position

Coyote skull -- foramen magnum is in the back, indicating spine is horizontal / Chimpanzee skull - foramen magnum is angled between bottom and back, indicating spine exits at an angle / Human skull -- foramen magnum is angled directly under or to the front, indicating spine is vertical.

In quadrupeds like a dog or coyote, the foramen magnum is right at the back of the skull -- thus when the dog walks on all fours, the head is pointed forward. Chimpanzees have a more upright posture. Like most apes, they walk on the knuckles of their hands, and their forelimbs are longer than the hind limbs (chimp skeleton showing knuckle-walking position). The foramen magnum must be more toward the bottom of the skull so that the spine can exit at an angle while the organism looks forward. Humans walk fully upright, with the spine more or less vertical. The foramen magnum in a human skull is all the way underneath, even angled slightly toward the front.

If intermediate fossils exist between apes and humans, what would you expect the position of the foramen magnum to be? Explain.

Please write your answer/response on the separate sheet.

Pelvis Anatomy

Another skeletal difference in bipeds is the shape of the hip bones. If you look at the chimp's skeleton again, you'll see that the hip bones are long and thin, and the flat blade (ilium) lies flat against the back. Looking downward along the axis of the spine, you can see this below.


Chimp hips. The blade of the ilium is flat against the back, that is, lies parallel to a line drawn across the back.

Human hips. The blade of the ilium wraps around the side of the body, almost perpendicular to a line drawn across the back.
Illustrations from Lovejoy, C.O. Evolution of human walking. Scientific American Nov. 1998, 118-125.
Human / Chimp

The gluteus maximus, gluteus medius and gluteus minimus muscles attach to the blade of the ilium in both humans and chimps. In the above illustration, the gluteus medius and gluteus maximus are labelled. Note that the gluteus maximus in both human and chimp is attached to the femur and to a point on the ilium in back of the body. When this muscle contracts, it pulls the femur up and back, propelling the body forward. The same is true of the gluteus medius and minimus in the chimp. In the human, though, the gluteus medius and minimus are attached to the part of the ilium that curves around the side of the body. What do they do? This is the lab portion of this course. Stand up. Yes, get out of the chair, right now.

Now, you're standing up. Put the heel of your right hand against your hip bone on the right side of your body. That's the ilium you feel there. Your fingers should be lying against the gluteus medius (maybe with a bit of fat on top of it -- see the diagram above). Now, raise your LEFT foot off the ground. Feel the RIGHT gluteus medius contract when you do this? Put your left foot down and pick it up a few times to feel how this works. Now you can sit down again.

When you lift one foot, the gluteus medius on the opposite side contracts, pulling the torso away from the side with the raised foot. If you didn't have a hip bone around the side there, when you lifted your left foot to take a step, you would lurch over to the left. Then when you took the next step with your right foot, you would lurch over to the right. If you've ever seen an ape walk on two legs, this motion should be familiar. You'd be walking like an ape. It's very inefficient, as a lot of the motion is side-to-side. When a human walks, the gluteus medius and minimus are constantly compensating for the shifting weight, so the motion is all forward. Of course, that leaves only the gluteus maximus to propel you forward, where the chimp uses all three muscles for that purpose.

If intermediate fossils exist between apes and humans, what would you expect their hip bones to be like? Explain.

Please write your answer/response on the separate sheet.

Of course, another difference between chimps and humans is that the cranium (brain case) of a chimp is about 400 ml in volume, while human cranial volume ranges from about 1200 to 1500 ml. If intermediate fossils exist between apes and humans, what would you expect their cranial volume to be like? Explain.

Please write your answer/response on the separate sheet.

The teeth of humans and apes are also different. Note below how large the canines are in the chimp compared to the rest of the teeth. Also, the arrangement of teeth in the jaw is U shaped in apes, and more C shaped in humans.

Chimpanzee upper jaw / Human upper jaw

If intermediate fossils exist between apes and humans, what would you expect their teeth and jaws to be like? Explain.

Please write your answer/response on the separate sheet.

One thing we haven't talked about yet is the mosaic character of evolution. That is, as organisms change over time, all their characteristics don't necessarily change at the same time. Thus, for instance, Archaeopteryx had wings and feathers, but its bone structure was almost identical to that of dinosaurs, and it had teeth. Instead of gradually changing everything at once, it looks more like bird ancestors changed things piecemeal, so that the intermediates are "mosaics" of reptilian and bird-like characteristics.

If ape-human intermediates were "mosaic" in this way, how would they differ from your descriptions above? Explain.

Please write your answer/response on the separate sheet.