Oligocene fossils explain
anthropoid distribution
The fossils found in Yunnan province,
China.Photo: Reuters
The climate change that occurred about 34
million years ago may hold the reason as
to why humans originated in Africa and
not in Asia. More specifically, the
deterioration of climate marking the
Eocene-Oligocene Transition (which
occurred about 34 million years ago)
could have acted as an evolutionary
filter, allowing different types of
primates to evolve in Africa as compared
to Asia. The ensuing dominance of the
subset of primates known as anthropoids
in African regions must have led to the
evolution of humans there, according to a
paper in Science. Therefore, even though
the earliest primate fossils have been
found in Asia, the actual evolution of
humans took place in Africa.
Geological calendar
The geological calendar relating to the
evolution of primates and humans
encompasses the so-called periods of
which two are of interest here — the
Paleogene and the Neogene. The Paleogene,
which lasted over 43million years,
beginning 66 million years ago (mya) and
ending about 23 mya, was the time when
mammals evolved from simpler forms. This
was followed by the Neogene period
(starting from the end of Paleogene to
about 2.5 mya) during which time early
humans evolved.
The Paleogene itself is divided into
three epochs: the Paleocene, Eocene and
Oligocene.
Nearly 55 mya, the earth suffered a
global increase in temperature of about 5
degrees Celsius. This is known as the
Paleocene-Eocene Thermal Maximum. During
this, the primates, from a subset of
which humans eventually evolved, being
highly sensitive to temperature changes,
migrated from their place of origin in
lower latitudes, to the northern land
regions. Subsequently, temperatures fell
during the Eocene-Oligocene Transition.
Marking this was a deterioration of
climate which caused these primates to
again retreat to lower latitudes.
Asian anthropoids
In a paper published in Science, Xijun Ni
and coworkers study a recently discovered
set of fossils of primates from the early
Oligocene period found in the Yunnan
province of southern China. These fossils
point to an African origin of humans in
the Neogene period, even though
anthropoids originated in Asia.
“The global climate deterioration, which
happened 34 million years ago, changed
the fate of anthropoids. We suggest that
this global climate deterioration is an
evolutionary filter for primates,” says
Xijun Ni in an email to this
correspondent.
The authors find that of the six fossils
studied, only one is an anthropoid.
Earlier known records of primate fossils
from the Late Eocene period from China,
Myanmar and Thailand have shown a
domination of stem anthropoids. So the
lack of anthropoids in the set of fossils
discovered now, which date later
geologically, could be attributed to the
filtering mechanism caused by lowering of
temperature and deterioration of climate
that marked the Eocene-Oligocene
Transition, the authors infer.
Dr Ni clarifies, “In Asia, this
[evolutionary] filter removed most of the
anthropoids but left lemur-like primates.
In Africa, the effect of this filter is
on the opposite side. It removed most of
the lemur-like primates but gave
anthropoids more opportunities. After the
filtering, the evolutionary center of
anthropoids moved from Asia to Africa.”
They also consider a fossil collection
from Pakistan, dated at a late earlyOligocene and find evidence of a few
primates. However, these were identified
as belonging to those species of smallbodied primates which did not evolve into
humans later.
In a complementary manner, the late
Eocene-early Oligocene primates from the
Afro-Arabian region had shown a very
different response to the EoceneOligocene Transition. There the
anthropoids diversified “taxonomically
and ecologically,” leading to the
eventual evolution of humans in the
Neogene period.
Source: xaam.in