Encyclopedia of Evolution.pdf - Online Reading Center

The initial divergence between hominins and the other
apes may have involved an abrupt genetic change. The
other great apes have 24 pairs of chromosomes in each cell,
while humans have just 23 pairs. The human chromosome
2 is the second largest human chromosome and resulted
from the fusion of two ape chromosomes. The hominins
that possessed this new chromosomal arrangement could
not interbreed with other apes and then evolved in their
own separate direction (see isolating mechanisms; speciation).
There are relatively few differences in the genes
that are on the chromosomes. Human and chimp DNA differs
by four percent. Just under 3 percent of the difference
is due to insertions or deletions (see DNA [raw material
of evolution]). Twenty-nine percent of the proteins are
identical in humans and chimps. bioinformatics studies
indicate that the major differences between humans and
chimpanzees are not in the genes that they have but the
genes that they use, especially in brain tissue. The human
lineage has evolved genetic differences that are important in
resistance to certain diseases. Humans and chimps also differ
markedly in which portions of the chromosomes experience
frequent recombination.
The earliest hominins represented numerous genera and
species, which can be lumped unofficially into a single category
(see australopithecines). The most important feature
that distinguished these australopithecines was bipedalism.
Australopithecines walked habitually on two legs, but many
retained some features of foot anatomy that allowed the continued
ability to climb trees. Australopithecines probably
could not run for long distances. Australopithecine bipedalism
was, however, no unstable transitional form between
chimp-like locomotion and modern human walking; it persisted
for more than a million years.
The earliest members of the genus Homo (two or more
species often lumped into Homo Habilis) possessed several
evolutionary innovations:
• Increase in brain size
• Production of simple stone tools (Oldowan technology)
• Reduction in tooth size
• More vertical face
These features were related to the consumption of meat, from
small prey or large carcasses. Fats and proteins from meat
were important in allowing larger brains to evolve, and larger
brains enabled the earliest humans to more effectively find
meat. Stone tools allowed the processing of meat, for example
to dismember carcasses so that the parts could be carried
home. Large teeth were no longer necessary for grinding
coarse plant materials, as meat constituted a larger portion
of the diet. To a large extent, stone tools made large teeth
unnecessary. The earliest Homo were taller than australopithecines,
but not as tall as modern humans.
The next phase of hominin evolution is represented by
several species (Homo erectus; Homo ergaster; Homo
HeiDelbergensis) often lumped into the species H. erectus.
H. ergaster was the African ancestor of Asian H. erectus and
of H. heidelbergensis, some populations of which migrated
to Europe. Asian H. erectus, as well as the Flores Island
hominin
people (H. floresiensis), became extinct, some as recently as
about 20,000 years ago. These hominins had no major evolutionary
innovations, but some of them had further development
of the unique features of Homo:
• Even larger brains
• More complex stone tools (Acheulean technology)
• More efficient bipedalism, allowing long-distance running
• Longer juvenile period
The nearly complete Nariokotome skeleton indicates that
H. ergaster was almost identical to modern humans except
for skull characteristics, particularly in having a brain that
was the largest that a primate had ever had but still much
smaller than the brains of modern humans. The rapid juvenile
brain growth required that birth occur earlier in the
period of gestation, otherwise the large head of the baby
would not have been able to emerge. Babies born in a more
helpless stage require longer parental care, a situation that
would have favored the evolution of a more intricate and
close-knit structure of immediate and extended families.
This situation, in turn, permitted the retention of juvenile
characteristics into adulthood (see neoteny). This may have
occurred first in H. ergaster.
Different Homo heidelbergensis populations followed
different evolutionary paths. In Europe, some H. heidelbergensis
populations evolved into Neandertals (H. neanderthalensis).
Neandertals had even larger brains (as large as
those of modern humans) and more advanced stone tools
(Mousterian technology). By about 130,000 years ago,
Neandertals were the only surviving European descendants
of H. heidelbergensis. In Africa some H. heidelbergensis
populations evolved into H. sapiens. H. sapiens had large
brains (of full modern size by 100,000 years ago) and may
have had more advanced stone tools and even art, as evidenced
by the Katanda harpoon point and the Blombos
Cave ochre stone. When H. sapiens coexisted with Neandertals
in the Middle East, both had the same Mousterian technology
of tools. The explosion of technology and art did not
occur until H. sapiens began to migrate out of Africa (see
Cro-Magnon).
The evolution of modern human characteristics followed
a mosaic pattern, in which different traits evolved at different
rates in different populations. In most australopithecines,
the face sloped more and the teeth were larger than in the
genus Homo. However, Sahelanthropus and Kenyanthropus
had relatively vertical faces, even while retaining other australopithecine
features. Kenyanthropus had smaller teeth than
other australopithecines. There was no single path of evolution
that all hominins followed.
The smaller teeth of Homo than those of earlier hominins
were not entirely due to eating more meat. The evolution
of larger brains allowed the development of technology
(tools and fire) for processing food and allowed the intelligent
location and selection of food. This allowed the evolution of
smaller teeth. In H. sapiens, the gut is smaller than what an
average primate of the same size would possess. By intelligently
selecting foods, H. sapiens had less need for digestive
detoxification.