"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 po
tassium-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. robustus . Though similar to the
Homo line in structure, their bones were thicker and more robust. These other hominids
are believed to have developed on a different lineage than the Homo line, and all of
these streams died out at around the time of Homo erectus, the next key hominid on the
human lineage. Because they are believed to have evolved apart from Homo hominids, it is
not important to cover these species in detail.
Homo erectus lived between 300,000 and 1,800,000 years ago, and still had portruding
jaws and a "keel" effect on the top of the cranium. It, like its predecessors, had no
definable chin, and thick brow ridges. However, skull capacity in these hominids jumped
from an average of 650 cm3 in H. habilis to an average of 900 cm3 in early specimens and
1100 cm3 in later specimens. The skeleton "is more robust than those of modern humans,
implying greater strength."(Foley, n.pag.) Due to their larger brain sizes, they are
believed to have posessed greater intelligence, and evidence of this has been found in
their probably use of fire, as shown by traces of burnt bones in cave floors, and the
finding of more sophisticated tools than H. habilis. They were shorter, on average, than
Homo sapiens, and their craniums showed a Nuchal torus, or a ridge, across the back of
the head. This species also had keeled craniums.
Archaic Homo sapiens, which first appeared 500,000 years ago, are believed to be our
most rescent relatives. By this time, the "keel" that existed on their skulls is
non-existent, and the supraorbital torus (the brow ridge) has begun to recede. Cranial
volume has been measured at an average of 1200 cm3, and their brain shape was probably
most similar to our own. Fossil evidence shows a trend for their posterior teeth to have
reduced in size, and the anterior teeth to have increased in size, from previous Homo
species, while late archaic Homo sapiens finds show a general reduction in the size of
both areas. The face and jaw areas also showed a reduction in size from previous
species.
It is at this point that Homo sapiens neanderthalensis enters the picture. Commonly
known as Neanderthal Man, this species is believed by most scientists to have existed at
the same time as late archaic Homo sapiens and early Homo sapiens sapiens, our own
species. Many scientists theorize that either we killed them off, or interbred with them
to produce modern humans. Their craneal volume is in fact higher than modern humans, at
an average of 1450 cm3. Their bones were also thicker, which implies greater bulk in
body. They also had larger nose cavities, a weak chin, and a portruding jaw area.
"Neandertals would have been extraordinarily strong by modern standards, and their
skeletons show that they endured brutally hard lives."(Foley, n.pag.) Neandertal
skeletons have been dated to between 30,000 and 230,000 years ago.
Finally, our own species is encountered. Scientists have dated the earliest Homo
sapiens sapiens fossils back 120,000 years. Our species showed an increase in skull
capacity up to an average of 1350 cm3. The supraorbital ridge is all but gone with
modern humans, and other features seen in earlier Homos, such as the "keel" and the
craneal ridges on the back are also gone. The cranium is more rounded, as opposed to the
general "pentagon" shape seen in earlier hominids. Teeth size for modern humans shows a
decrease in size from archaic Homo sapiens. Also, bone size shows a trend towards
reduced robustness, with thinner bones and smaller jaws.
From all the fossil evidence, a rough line can be drawn for human evolution, starting
from A. afarensis and ending in H. sapiens sapiens. A clear progression of features,
especially in the cranial region, can be seen. Features such as brain size are seen to
have developed and increased from our earliest ancestors up until now, while other
"non-essential" features, like a furry skin, a supraorbital ridge, and large teeth, have
diminished. This shows evolution of our species, from a more primitive creature, to our
modern shape, which is highly adaptive, intelligent, and suited to any environment. God
has created the perfect creature - a creature that evolves to suit its needs.
Bibliography
archaic - obsolete, antique. In hominid terms, it is used to describe Homo sapiens,
because they existed before out own species, Homo sapiens sapiens, with a similar name.
cm3 - cubic centimeter, a measurement of volume. Also known as a "cc."
cranial volume - also, "cranial capacity," or "skull capacity." Refers to the volume of
the area within the cranium, which is a rough (but not exact) indicator of an organism's
brain size.
cranium - the skull, referring especially to its upper section, which held the brain.
hominid - a human-like species. Hominids are identified by the ability to walk upright,
and a general internal structure similar to our own. The first believed hominid is
Australopithecus boisei, which existed between 1.1 and 2.1 million years ago.
phylogeny - a depiction of the evolutionary lines of descent for a particular organism -
the "family tree" of a species.
supraorbital - above the eye socket
Works Cited
Eldredge, Niles. Life Pulse: Episodes From the Story of the Fossil Record. pp. 233-240.
New York: Facts On File Publications, 1987.
Foley, Jim. "Hominid Species." The Fossil Hominids FAQ. 1996. On-line. Internet. 1
Jan. 1997. Available: http://earth.ics.uci.edu:8080/faqs/homs/species.html.
Leaky, L.S.B. The Progress and Evolution of Man in Africa. Toronto: Oxford University
Press, 1961.
Johanson, Donald and Edey, Maitland. Lucy: The Beginnings of Humankind. New York: Simon
& Schuster, 1981.
Rak, Yoel. The Australopithecine Face. New York: Academic Press, 1983.
Stoner, Don. A New Look At An Old Earth. June, 1992. On-line. Internet. 11 Jan.
1997. Available: http://www.power.net /users/aia/newlook/NLCHPTR5.HTM#Top.
Turnbaugh, William A., et al., Understanding Physical Anthropology and Archaeology,, 5th
Edition. Minnesota: West Publishing Company, 1993.
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