“You can think of this as a ball and chain attached
to the leg of every coral larva.”—chris LAnGdon
indian ocean set stage for humans
ing the last several million years, drilled
across the Indian, Pacific and Atlantic
oceans. Most strikingly, a site in
the eastern Indian Ocean and another
in the western Indian Ocean show
temperatures tracking similarly until
around 2 million years ago, when the
eastern site warmed and the
western site cooled. If so,
then the cool waters off East
Africa would have dried out
that part of the continent,
deMenocal said.
Next the scientists ran
computer simulations that
erased the east/west temperature difference and
showed what might happen if temperatures were
the same along the equator
across the Indian Ocean.
That change shut down a type of atmospheric circulation, making East Africa
wetter than it is today.
Taken together, deMenocal said, “I
think this is pretty solid evidence for a
transition to more open conditions [in
East Africa] at this time.” What kicked
off the change in ocean temperatures,
though, remains a mystery.
Other, more recent climate changes
may have also shaped the course of
human evolution, said Andrew Weaver,
a climate scientist at the University of
Victoria in Canada. At the conference
he reported new simulations looking at
climate changes that happened about
105,000 years ago, around the time modern humans, Homo sapiens, were moving
out of Africa.
When the northern ice sheets dumped
great icebergs into the ocean, freshening
the water there, rain belts in Africa also
shifted, Weaver reported. In this case,
the change may have dried out much of
northern Africa, compelling H. sapiens
to leave its birth continent. s
More rain
occurs where
the ocean
is warmest,
because
water can
more readily
evaporate
and fall back
as rain.
by Alexandra Witze
Scientists may now be able to explain
one of the key events that shaped human
history: why East Africa got drier starting around 2 million years ago, with
forests giving way to grasslands on
which the genus Homo
further evolved. Ocean
temperature changes, especially the arrival of a strong
warm/cool difference along
the equator in the Indian
Ocean, could have triggered
the change.
“Those gradients are
responsible for shifting
rainfall towards or away
from East Africa,” said Peter
deMenocal, a paleoclimatologist at the Lamont-Doherty
Earth Observatory in Palisades, N. Y. He
presented new details about his idea on
February 17.
Researchers infer that East Africa
started shifting toward grasslands by
looking at the proportion of fossils of
grazing animals, which peaked around
1. 5 million years ago. Around this time
Homo began to develop ne w tools, diversify into new species and make its first
tentative forays out of Africa.
But despite a raft of theories, scientists haven’t been able to explain what
triggered the drying responsible for
the shift to grasslands. The ocean gradient idea might do the trick, deMenocal
said.
“At first blush it doesn’t seem
intuitively obvious, but what controls
rainfall in the tropics is where the warm
water is,” he said. More rain occurs
where the ocean is warmest, because
water can more readily evaporate and
fall back as rain.
DeMenocal and his colleagues looked
at deep-sea sediment cores, represent-
fracking isn’t tainting
groundwater, study finds
Several communities have
reported groundwater contami-
nation associated with natural
gas produced from deep shale
deposits by hydraulic fracturing,
sometimes called fracking. a new
study by researchers at the univer-
sity of Texas at austin concludes
that such pollution results not
from liquids pumped down to the
shale deposits to fracture the deep
rock — as often claimed — but
instead from poor management
of wastes or bore wells thousands
of feet above the shale. Funded
by the university with no industry
contributions, the study doesn’t
downplay drinking-water pollution
associated with shale-gas produc-
tion. But those risks “are very
similar, if not exactly the same, as
the impacts that we see from con-
ventional gas development,” said
charles “chip” Groat, who led the
study. Groat reported his team’s
findings February 16.
— Janet Raloff
Lab-grown meat almost done
Hamburger made from meat
grown in the lab might be ready
to eat this october, Mark Post
from Maastricht university in the
netherlands reported February
19. So far, scientists using bovine
stem cells have made pieces of
skeletal muscle that are about 3
centimeters long. citing the grow-ing global demand for meat and
the environmental costs of raising
livestock, Post said “we need to
get alternatives.” He thinks that
lab-grown meat products could be
commercially available in 10 to 20
years. — Rebecca Cheung
www.sciencenews.org
March 10, 2012 | SCIENCE NEWS | 11
