Human Evolution and the Fossil Record

Scientists continue to debate the history of man. It is generally
agreed upon by the scientific community, however, that humans evolved from
lesser beings, and this essay will function to provide evidence to support this
claim. Several points will be outlined, including the general physical changes
that occurred between several key species on the phylogeny of man, and a
discussion of dating methods used to pinpoint the age of the fossils.
This essay will begin with a brief discussion of dating techniques. In
the study of hominid evolution, two main methods of dating are used: carbon-14
and potassium-argon dating. Carbon-14 dating involves the decay of radioactive
C-14, which has a half life of 5770 years. This makes this method useful for
dating of recent fossils, with good accuracy, up to 50,000 years back. After
5770 years, half of the carbon-14 in a fossil decays to nitrogen-14. Since the
ratio of carbon-12 to carbon-14 in a living organism remains the same as in the
environment around them because the organism constantly eats and replenishes it,
if it were to die, the ratio would change greatly after many years. It is the
difference between this ratio now and the time is died that allows a date for it
to be established. Potassium-argon dating, another dating method, is possible
due to volcanic ash and rocks found near many fossil sites. Rocks and ash
created in this manner contain potassium-40, but no argon. As time passes, the
potassium-40 decays into argon-40. In the laboratory, the sample is reheated,
and since argon-40 is a gas, it is released. The ratio of argon-40 released to
potassium-40 still present allows for a date to be assigned to objects near the
sample. However, due to potassium\'s high half-life (1.3 billion years), it is
only useful as a dating technique for finds older than 500,000 years old. Also,
it is only useful where volcanic activity existed. Both these methods have
error margins, ranging from a few thousand years in carbon-14 dating to tens of
thousands of years, or more, for potassium-argon dating. However, thanks to
scientific breakthroughs, these two processes can be used with reasonable
security in establishing a time for fossils.
Our farthest believed ancestor is believed to be Australopithecus
afarensis. This species, which lived between three and four million years ago,
is believed to be the first real hominid because it is the oldest, and "most
primitive of any definite hominid form thus far found."(Turnbaugh, 281) Evidence
from fossilized footprints, as well as pelvic and leg bones which were similar
to modern hominids, led scientists to believe that they could walk upright. Its
teeth resembled more those of primates, due to their large size. Its skull
capacity ranged from 350 to 500 cm3. This species, though it had some hominid
characteristics, was still more like an ape. Its face portruded outwards near
the mouth region, and it did not have a definable chin. Finally, their craniums
had large, portruding ridges over either eye.
Another important being in the human timeline is Australopithecus
africanus. Many scientists believe that it is the next in the sequence leading
to man, however, a few believe that it belongs to a lineage on its own. A.
africanus fossils have been dated back to the time period between two and three
million years ago. It had a greater body size than A. afarensis, and a skull
volume ranging between 420 and 500 cm3. It averaged a little higher in height
than the 3 to 5 feet believed for A. afarensis. Its jaws also portruded out.
The "keel" effect is very distinguishable on this species, as it is with many of
the older hominid species - a slight peak on the top of the cranium. Ridges
over the eyes were also prominent on this hominid.
The next species believed to be in our line of descent is Homo habilis.
This is the first being with the distinction of having Homo as its genus. This
species, which is dated back to between 1.5 and 2.4 million years ago, had a
face which portruded less than A. africanus and A. afarensis. Its teeth, though
still larger than modern humans, were smaller than those of its ancestors.
Finally, its fossil fragments displayed "an average increase in cranial size of
21 percent and 43 percent, respectively, over [A. africanus and A.
afarensis],"(Turnbaugh, 288) with an average cranial capacity of 650 cm3.
Skulls found of this hominid also feature a bulge of "Broca\'s area," an area
essential for human speech. It was also taller than the previous hominids,
averaging around 5 feet high.
At about the same