MARGINALIA
Hunting the First Hominid
Pat Shipman
Self-centeredly, human beings have always taken an exceptional
interest in their origins. Each discovery of a new species of
hominid—both our human ancestors and the near-relatives
arising after the split from the gorilla-chimp lineage—is
reported with great fanfare, even though the First Hominid remains
elusive. We hope that, when our earliest ancestor is finally
captured, it will reveal the fundamental adaptations that make us
us.
There is no shortage of ideas about the essential nature of the
human species and the basic adaptations of our kind. Some say
hominids are fundamentally thinkers; others favor tool-makers or
talkers; still others argue that hunting, scavenging or bipedal
walking made hominids special. Knowing what the First Hominid looked
like would add some meat to a soup flavored with speculation and
prejudice.
Genetic and molecular studies provide one sort of insight, showing
what sort of stuff we are made of, and that it is only
slightly different stuff from that which makes up the apes
(gorillas, chimps, orang-utans and gibbons). From the molecular
differences among the genes of humans and apes, geneticists estimate
the time when each of the various ape and hominid lineages diverged
from the common stem. The result is a sort of hairy Y diagram, with
multiple branches instead of simply two as is usual on a Y. Each
terminus represents a living species; each branching point or node
represents the appearance of some new evolutionary trait, such as
new molecules, new genes, new shapes or new proportions of limb,
skull and tooth. Unfortunately, this way of diagramming the results
tends to lull us into thinking (falsely) that all the evolutionary
changes occurred at those nodes, and none along the branches.
Such studies omit a crucial part of our history: the extinct
species. Only the fossil record contains evidence, in context, of
the precise pathway that evolution took. In this technological age,
when sophisticated instrumentation and gee-whiz algorithms seem
downright necessary, the most basic information about the evolution
of our lineage still comes from branches of science that operate in
rather old-fashioned ways. The primary discoveries in paleontology
(the study of old things), paleoanthropology (the study of old
humans) and archaeology (the study of the old stuff that old humans
leave around) still rely on the efforts of a handful of
investigators who slog around on foot or excavate in desolate
landscapes. Fancy equipment can't replace eyes and brains, although
instrumentation plays a crucial role in the dating and analysis of
fossil remains.
Finding the evolutionary origin of hominids is a little like
stalking big game. Paleoanthropologists struggle to establish when
and where their quarry was last seen and what it was
like—hoping to follow its tracks backward in time. (Why
hominids or any other group arose is such a metaphysical
question that most paleoanthropologists run away screaming when it
is asked.)
When the first hominid slinked through the underbush has
been estimated from molecular clocks and confirmed by radiometric
dates. These lines of evidence converge on a period between 5
million and 7 million years ago as the time when a primitive,
perhaps vaguely apelike species evolved some definitive new
adaptation that transformed it into the First Hominid. But like the
point of inflection on a line graph, the first species in any new
lineage is only readily apparent after the fact. The emergence of
the first hominid was probably not obvious in prospect but only now,
in retrospect—in the context of the entire evolutionary record
of the hominids—when the long-term evolutionary trends can be
seen.
Where this dangerous creature once lived has to be Africa,
since both our closest living relatives (chimpanzees and gorillas)
and all early hominids (older than about 2 million years ago) are
African.
» Post Comment