Members of three
different hominid lines clustered
at the bottom of Africa around 2 million
years ago, signaling an evolutionary swing propelled by the spread of a highly
successful, humanlike species, new fossil discoveries suggest. It’s unclear,
though, if the three ancient populations inhabited the region at precisely the
same time.

Excavations at Drimolen, a set of caves in South Africa,
uncovered two fossil braincases, one from Homo
and the other from Paranthropus
, say paleoanthropologist Andy Herries of La Trobe University in Melbourne,
Australia, and his colleagues. Both finds date to between 2.04 million and 1.95
million years ago, the scientists report in the April 3 Science.

The H. erectus
fossil comes from a child who displayed a long, low braincase typical of adults
from that species. The P. robustus
braincase is that of an adult.

Researchers previously determined that two Australopithecus species, A. africanus and A. sediba (SN: 7/25/13),
inhabited nearby parts of South Africa approximately 2 million years ago.

Taken together, these discoveries indicate that a major
transition in hominid evolution occurred in southern Africa between around 2.1
million and 1.9 million years ago, Herries’ team says. During that stretch,
climate and habitat fluctuations drove Australopithecus
species to extinction. H. erectus and
P. robustus weathered those
ecological challenges, possibly outcompeting Australopithecus for limited resources, the researchers speculate.

It’s unclear whether members of the three hominid lines ever
encountered each other during that transition period.

Drimolen excavation site
Excavators at South Africa’s Drimolen site, shown with some of the animal bones they have found, have recovered roughly 2-million-year-old fossils of two hominid species, Homo erectus and Paranthropus robustus.A. Herries

“These spectacular discoveries confirm what some of us have
expected for some time, that three genera of [hominids] coexisted in southern
Africa,” says paleoanthropologist Darryl de Ruiter of Texas A&M University
in College Station, who was not involved in the research.

Earlier work at several other South African cave sites had
suggested that H. erectus, P. robustus and A. sediba all dated to nearly 2 million years ago. But many fossils
from the first two species are fragmentary, and precise dating of cave
sediments that held those finds has proven difficult.

Herries’ team dated the fossil braincases at Drimolen using
two techniques for calculating the time since sediments formed just below and
above where the specimens were found. Evidence of previously dated reversals of
Earth’s magnetic field in Drimolen sediment helped to confirm age estimates for
the fossils.

The South African H.
fossils may be slightly older than those of A. sediba, but a controversial proposal that A. sediba was an ancestor of the Homo genus remains in play, de Ruiter says. Researchers don’t know how
much earlier than 2 million years ago A.
originated or how far it ranged beyond its one known fossil site in
South Africa. Even so, some other researchers consider A. sediba a dead-end species and regard East Africa as the best bet
for where Homo originated.

Unearthing an H.
fossil dating to around 2 million years ago in South Africa
considerably expands that species’ range at an early stage of its evolution,
says paleoanthropologist John Hawks of the University of Wisconsin–Madison.
H. erectus fossils in western Asia date
to about 1.8 million years ago (SN: 10/17/13).
And H. erectus may have made
2.1-million-year-old stone
tools in China
(SN: 7/11/18).

“It’s possible that this child from Drimolen is the
earliest-known representative of the first global [hominid] species,” says Hawks,
who did not participate in the new study.

H. erectus’ last known appearance
was as late as 108,000 years ago on an Indonesian island, meaning it survived
about 2 million years (SN: 12/18/19).

The H. erectus
fossil found at Drimolen “marks the beginning of the most successful species of
Homo ever known — present company
included,” writes paleoanthropologist Susan Antón of New York University in a commentary
published with the new Science report