, 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. 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
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http://earth.ics.uci.edu:8080/faqs/homs/species.html.
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