hunting for humans

first_human_coverBack when I took anthropology, I learned an orderly evolutionary line of succession from australopithecines (“Lucy”), to homo habilis, to homo erectus, to Neandertals and homo sapiens sapiens. That line was too tidy. Besides ignoring various branches of both australopithecus and homo, anthropologists have long debated whether Lucy, at ~3 million years old, represents the earliest human ancestor after the split from what would become chimpanzees. Still others argue that australopithecines are a separate evolutionary track that never evolved into humans in the first place. Hence the book’s title: what, or who, was the first human?

One way to try and answer that question is to examine DNA. Because genetic mutations occur at a constant rate, the accumulation of differences in DNA can act as a “molecular clock” to estimate when two species last shared an ancestor. The molecular clock places the human-chimp split at nearly 7 million years ago, more than twice as old as Lucy.

Fossils point the same direction. One major fossil find was ardipithecus ramidus, which is thought to have lived 4-5 million years ago. The primary a. ramidus fossil is a skull found in the early 1990s but not reported until nearly 15 years later. Why? The skull, though nearly complete, had been flattened to less than an inch tall by the time it was dug up. Demonstrating admirable patience, complete insanity, or the consequences of an unending supply of ritalin, a researcher spent more than a decade un-flattening it.

A. ramidus is interesting in part because it may upset long-held notions about the evolution of bipedalism. According to the savannah hypothesis, mankind’s shift to bipedalism came when our ancestors left the treetops and entered a mostly treeless savannah, where an upright posture was that much more useful, such as peeking over the top of tall grasses. The woodland hypothesis, in contrast, holds that a. ramidus fossils come from woodlands—suggesting that our ancestors didn’t move directly from trees to savannah, but hung out in the woods for awhile. And since the move to the savannah came later, it couldn’t have been the impetus for bipedalism. If it’s true, one consequence of the woodland hypothesis is that we don’t find a lot of hominid fossils in part because we’re looking in the wrong places.

An older hominid species is Orrorin tugenensis, which at more than 7 million years old fits the molecular clock estimate for the first human. O. tugenensis is unique because almost all of the fossils left behind are post-cranial (below the neck); the primary fossil is a large piece of femur. For a species to qualify as the “first human,” it must be a hominid. Hominid doesn’t have a hard-and-fast definition, but O. tugenensis checks off a lot of boxes: flattened (rather than pointed) canines, a flatter face, less of a brow ridge, greater cranial capacity, the U vs. V shape formed by the teeth, the shape of the molars, the position of the spine’s connection to the skull.*

*All that evidence comes from a minuscule number of fossils, most of which are 2-inch bits of jawbone. Anthropologists are real gluttons for punishment in that regard, especially considering how paleontologists can find 90% complete skeletons of 80-million year old dinosaurs.

Sahelanthropus tchadensis is a species known only from a partial skull, and its discoverers argue that it is the oldest known human ancestor, post-chimp-split. Unfortunately, it is not known whether S. tchadensis walked upright, which is a classic criterion for hominidism. The best evidence is the angle at which the spinal cord joins the skull, which in S. tchadensis is more typical of bipedalism. When looking at bipedalism, two important factors are at play: first, if the evidence comes from lower-body structures (e.g., legs, hips), or is intuited from cranial and dental evidence (a so-called “dental hominid”). Second, whether an upright walking style is “human-like” or “chimp-like”—in other words, whether someone actually was bipedal, or if it just could be briefly bipedal the way that chimps can. Most accept that tchadensis was more bipedal than quadruped, but there’s still debate about whether its bipedalism was humanlike.

While these fossils are all older than Lucy, the evolutionary picture is still unclear, but certainly much more treelike than linear, as demonstrated in this wonderful Smithsonsian exhibit.

Advertisements

One thought on “hunting for humans

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s