Human Evolution And The Fossil Record Essay

, Research Paper

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 time as Homo habilis and some of the other Homo

species, other hominid species belonging to the Australopithecus genus, are

believed to have co-existed. These include A. robustus, A. boisei, A.

aethiopicus, and A. ro

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