Last year, scientists announced that a human
jawbone and prehistoric tools found in 2002 in Misliya Cave, on the
western edge of Israel, were between 177,000 and 194,000 years old.
The finding suggested that modern humans, who originated in Africa,
began migrating out of the continent at least 40,000 years earlier than
scientists previously thought.
But the story of how and when modern humans originated and spread
throughout the world is still in draft form. That's because science
hasn't settled how many times modern humans left Africa, or just how
many routes they may have taken.
A new study published this week [Nov. 25, 2019] in the
Proceedings of the National Academy of Sciences
by American and Israeli geoscientists and climatologists provides
evidence that summer monsoons from Asia and Africa may have reached into
the Middle East for periods of time going back at least 125,000 years,
providing suitable corridors for human migration.
The likely timing of these northward monsoon expansions corresponds
with cyclical changes in Earth's orbit that would have brought the
Northern Hemisphere closer to the sun and led to increased summer
precipitation. With increased summer precipitation there may have been
increased vegetation, supporting animal and human migration into the
region.
"It could be important context for experts studying how, why, and
when early modern humans were migrating out of Africa," says lead author
Ian Orland, a University of Wisconsin-Madison geoscientist now at the
Wisconsin Geological and Natural History Survey, in the Division of
Extension. "The Eastern Mediterranean was a critical bottleneck for that
route out of Africa and if our suggestion is right, at 125,000 years
ago and potentially at other periods, there may have been more
consistent rainfall on a year-round basis that might enhance the ability
of humans to migrate."
For as long as humans have kept records, winters have been wet and
summers have been hot and dry in the Levant, a region that includes
Israel, Syria, Lebanon, Jordan and Palestine. Before modern times, those
hot, dry summers would have presented a significant barrier to people
trying to move across the landscape.
Scientists, though, have found it difficult to determine what kinds
of precipitation patterns might have existed in the prehistoric Levant.
Some studies examining a variety of evidence, including pollen records,
ancient lake beds, and Dead Sea sediments, along with some climate
modeling studies, indicate summers in the region may have, on occasion,
been wet.
To try to better understand this seasonality, Orland and colleagues
looked at cave formations called speleothems in Israel's Soreq Cave.
Speleothems, such as stalactites and stalagmites, form when water drips
into a cave and deposits a hard mineral called calcite. The water
contains chemical fingerprints called isotopes that keep a record, like
an archive, of the timing and environmental conditions under which
speleothems have grown.
Among these isotopes are different forms of oxygen molecules -- a
light form called O16 and a heavy form called O18. Today, the water
contributing to speleothem growth throughout much of the year has both
heavy and light oxygen, with the light oxygen predominantly delivered by
rainstorms during the winter wet season.
Orland and his colleagues hypothesized they might be able to discern
from speleothems whether two rainy seasons had contributed to their
growth at times in the past because they might show a similar signature
of light oxygen in both winter and summer growth.
But to make this comparison, the scientists had to make isotope
measurements across single growth bands, which are narrower than a human
hair. Using a sensitive instrument in the UW-Madison Department of
Geoscience called an ion microprobe, the team measured the relative
amounts of light and heavy oxygen at seasonal increments across the
growth bands of two 125,000-year-old speleothems from Soreq Cave.
This was the first time that seasonal changes were directly measured in a speleothem this old.
At the same time that Orland was in pursuit of geologic answers, his
UW-Madison colleague in the Nelson Institute for Environmental Studies
Center for Climatic Research, Feng He, was independently using climate
models to examine how vegetation on the planet has changed with seasonal
fluctuations over the last 800,000 years. Colleagues since graduate
school, He and Orland teamed up to combine their respective approaches
after learning their studies were complementary.
A previous study in 2014 from UW-Madison climatologist and Professor
Emeritus John Kutzbach showed that the Middle East may have been warmer
and wetter than usual during two periods of time corresponding roughly
to 125,000 years ago and 105,000 years ago. Meanwhile, at a point in
between, 115,000 years ago, conditions there were more similar to today.
The wetter time periods corresponded to peak summer insolation in
the Northern Hemisphere, when Earth passes closer to the sun due to
subtle changes in its orbit. The drier time period corresponded to one
of its farthest orbits from the sun. Monsoon seasons tend to be stronger
during peak insolation.
This provided He an opportunity to study high and low insolation
rainfall during summer seasons in the Middle East and to study its
isotopic signatures.
The climate model "fueled the summer monsoon hypothesis" because it
suggested that "under these conditions, the monsoons could have reached
the Middle East and would have a low O18 signature," He, a study
co-author, says. "It's a very intriguing period in terms of climate and
human evolution."
His model showed that northward expansion of the African and Asian
summer monsoons was possible during this time period, would have brought
significant rainfall to the Levant in the summer months, would have
nearly doubled annual precipitation in the region, and would have left
an oxygen isotope signature similar to winter rains.
At the same time, Orland's speleothem isotype analysis also
suggested summers were rainier during peak insolation at 125,000 and
105,000 years ago.
For similar reasons, the Middle East may have also been warm and
humid around 176,000 years ago, the researchers say -- about when the
jawbone made its way to Misliya Cave. And before the jawbone, the
previous oldest modern human fossils found outside of Africa were at
Israel's Skh?l Cave, dating back between 80,000 and 120,000 years ago.
Overall, the study suggests that during a period of time when humans
and their ancestors were exploring beyond the African continent,
conditions may have been favorable for them to traverse the Levant.
"Human migration out of Africa occurred in pulses, which is
definitely consistent with our idea that every time the Earth is closer
to the sun, the summer monsoon is stronger and that's the climatic
window that opened and provided opportunities for human migration out of
Africa," says He.