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evolutionary ecologist John Stewart of
Bournemouth University in England
propose that the evolution of humans
and earlier Homo species that reached
Asia and Europe hinged on small populations that took refuge in ecological
sanctuaries during recurring ice ages.
As with many plants and animals, cli-mate-induced corralling of Homo groups
into restricted habitats prompted the
evolution of new species, most notably
Neandertals and Denisovans, Stringer
and Stewart hypothesize. Their new
work lays out a framework of what early
human migration and evolution might
have looked like.
Consider Homo heidelbergensis, a likely
ancestor of Neandertals and H. sapiens
that left Africa between 600,000 and
400,000 years ago. H. heidelbergensis
apparently survived in livable parts of
southwestern Asia during ice ages. A
long-isolated H. heidelbergensis popu-
lation evolved into Neandertals, Stringer
suggests.
Back Story | SKULL TO SKULL
Although DNA analyses have allowed the identi;cation of distinct hominid species
like the Denisovans, human evolutionary scientists still rely largely on fossil studies
to reveal differences between early human and humanlike species. A comparison
of the skulls of contemporaries Homo neanderthalensis and Homo sapiens, for
example, argues that these were indeed two separate species, both thought to have
evolved from the more ancient Homo heidelbergensis. Scientists look to certain key
features when evaluating such fossils.
Homo neanderthalensis
Homo sapiens
Skull The Neandertal skull is thicker,
with a round protrusion at the back of
the head. H. sapiens’
skull has a more
rounded dome
shape.
Braincase
The Neandertal braincase is slightly larger.
Face H. sapiens has
a relatively ;at, small
face. Neandertals had
a sloping face.
Forehead
The Neandertal forehead recedes while
H. sapiens has a high,
distinctive forehead.
Brow ridges
Protruding brow
ridges distinguish
the Neandertal.
Chin H. sapiens
has a well-developed
chin.
Longlin
Maludong
New species? Fossils unearthed from two
caves in China include a skull that combines
modern human facial features with those of
earlier Homo species (reconstruction shown).
evidence suggests that H. sapiens and
its African ancestors originated during
volatile periods — lasting up to 300,000
years — when the climate veered from
extremely rainy to bone-dry every 8,000
to 12,000 years, Potts says.
Faced with constantly shifting habitats, ancient people and their precursors
evolved to deal with a wide range of
environments, he asserts. East Asian
migrants would have adapted to ice ages
rather than seeking ecological shelter.
Anthropologist David Frayer of the
University of Kansas in Lawrence also
doubts that cold-weather refuges stoked
human evolution. In fact, he goes as far
as to argue that Stone Age groups in Asia
and Europe interbred enough, even during ice ages, to maintain H. sapiens as a
widespread, diverse species that encompassed both Neandertals and the newly
reported Chinese individuals.
Consistent with Stringer’s argument,
an analysis of ancient DNA, published
online February 23 in Molecular Biology
and Evolution, suggests that one portion
of the Neandertal population sought ice
age sanctuary in Western Europe between
70,000 and 55,000 years ago before dying
out and giving way to remaining Neandertals from Eastern Europe. Geneticist
Love Dalén of the Swedish Museum of
Natural History in Stockholm and his
colleagues found much less genetic diversity in Western European versus Eastern
European Neandertals.
Such evidence may point toward ref-uge-driven evolution outside Africa, but
much remains unknown. “Asia is huge
and we know little about ancient human
populations there,” Stringer says. s
MAP: GEOATLAS/GRAPHI-OGRE, ADAPTED BY E. FELICIANO; RECONSTRUCTION: PETER SCHOUTEN; SKULLS: HUMAN ORIGINS PROGRAM, SMITHSONIAN INSTITUTION
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