MY AMERICAN SCIENTIST
LOG IN! REGISTER!
SEARCH
 
RSS
Logo IMG

MARGINALIA

Hunting the First Hominid

Pat Shipman

What to Look For?

What happened, exactly, and to whom, remain to be discovered. Two newly discovered fossil species have each been proposed to be the First Hominid. This circumstance raises a significant issue: How would we know the First Hominid if we saw it?

Making a list of key features that differentiate apes from people is not difficult, but misleading. It is absurd to expect that all of these differences arose simultaneously during a single evolutionary event represented by the final fork on the hairy Y diagram. The first ape on the gorilla-chimp lineage was neither a gorilla nor a chimpanzee, for modern gorillas and chimps have had at least 5 million years to evolve in isolation before arriving at their modern form. In the same way, the First Hominid on our lineage was not a human and did not possess all of the characteristics of modern humans.Figure 1. <em>Ardipithecus ramidus kadabba</em> fossils . . .Click to Enlarge Image

Using a hairy Y diagram, we can limit the number of contenders for the essential or basal hominid adaptation:

—Hominids might be essentially bipeds. All known hominids are bipedal; no apes are.

—Wishful thinking aside, hominids are not simply brainy apes. Alas, until about 2 million years ago no hominid had a brain larger than an ape's relative to its body size.

—Hominids might be apelike creatures that have lost their sexual dimorphism. Sexual dimorphism is exhibited as male-female differences that are not related to reproduction. For example, male orangs are typically much larger than females and have longer canine teeth that hone sharper with wear. The fossil record shows that hominids lost their dental sexual dimorphism first, since all known hominids have small and flat-wearing canines. In contrast, sexual dimorphism in body size persisted in hominids until about 2 million years ago, when the genus Homo first appeared.

—Thick dental enamel may be a key hominid trait. All hominids have thick enamel, whereas all fossil and living apes (except those in the orang-utan lineage) have thin enamel. Because the fossil record of apes is so poor, we do not know whether the primitive condition for apes and hominids was thick or thin enamel. Indeed, how enamel thickness is to be measured and evaluated has generated many pages of debate.

—Hominids are hand-graspers or manipulators (from the Latin for hand, manus), whereas apes are foot-graspers. These differences are reflected in the sharp contrasts in the hand and foot anatomy of apes and humans. Apes have divergent big toes and long, curved toe bones for holding onto branches; their thumbs are short and cannot be opposed to the other fingers for skillful manipulation. Human beings are the opposite, with long, opposable thumbs and big toes that are closely aligned with the remaining short, straight toes. Human feet are nearly useless for grasping but are well adapted to bipedalism. An intermediate condition occurs in early hominids such as Lucy (the best-known individual of Australopithecus afarensis), who had opposable thumbs and numerous adaptations to bipedalism, and yet retained rather long and curved toes. Lucy and probably other types of Australopithecus were walkers, hand-graspers and somewhat compromised foot-graspers.

A description of our desired prey, then, might read like this:

An ape-brained and small-canined creature, with dental enamel of unknown thickness. Large if male but smaller if female. May be spotted climbing adeptly in trees or walking bipedally on the ground. Last seen in Africa between 5 million and 7 million years ago.

From this description, can we identify the First Hominid? Well, no—not yet.





» Post Comment

 

EMAIL TO A FRIEND :

Of Possible Interest

Spotlight: Briefings

Letters to the Editors: Only Human

Science Observer: In the News

Subscribe to American Scientist