Asked and answered counselor.
From the American Association for the Advancement of Science's ScienceInsider blog:
Researchers have often proposed that dramatic changes in ancient climates triggered major events in human evolution, such as the emergence of a new species or migrations of our ancestors in and out of Africa. But it has been notoriously difficult to tie deep-sea records of global climate fluctuations to local fossil sites in Africa and, hence, to specific fossils of human ancestors. Now, a new report released yesterday by the National Research Council recommends a major new interdisciplinary research program to study how past climate influenced human evolution.
In the report, an interdisciplinary team of paleoanthropologists and geologists recommended four new research initiatives over the next 10 to 20 years. The first is to launch a major effort to locate new fossil sites using remote-sensing tools and traditional ground methods to survey new terrain. The point is to fill in key gaps in the fossil record, such as when new species first appear and disappear, to see if there are links between these major events in human evolution and changes in the climate.
The researchers also called for a comprehensive program to drill ancient lakebeds and lakes on land, as well as in ocean basins, in the regions where humans evolved in the Rift Valley of Ethiopia and Kenya, for example. This would help provide a record of climate changes in the local areas where human ancestors actually lived, given that the climate can vary dramatically in different parts of Africa. It would be part of a larger effort to reconstruct past environments in local habitats where fossils have been found....
From PBS' Nova series "Becoming Human". This extended excerpt is the end of part one (transcript):
...BRIAN RICHMOND: What we see in the evolution of Homo habilis is an expansion in the brain size compared to Australopithecus. So here is the skull of Australopithecus, and it has no forehead, it just has a straight slope behind the orbits. Whereas here, in Homo habilis, you see a sloping, elevated forehead. And in Australopithecus the area behind the orbits is pinched in, also reflecting a small frontal region.
In contrast, in Homo habilis, we see an expansion of that area behind the orbits that points to an expansion in the cognitive capabilities of higher functions, of higher reasoning functions of the brain.
NARRATOR: It was an expansion equivalent to a doubling of brain volume.
RALPH HOLLOWAY: Once you go from something like 400 ccs in Australopithecines to, say, 700, 800 ccs in Homo habilis, yes, you're getting, getting a big increase in cognitive capacity.
NARRATOR: And along with his bigger brain, Homo habilis was starting to look a lot more human.
The contours of fossil skulls allow reconstructionist Viktor Deak to reveal the faces of early human beings.
Gone is the projecting snout of an ape. In Homo habilis the face of humanity is emerging. This poses a great enigma: why, after millions of years of flat-lining did brain size and mental capacity suddenly take off? Two million years ago, what jump-started human evolution?
Scientists all over Africa looked for clues.
Here in Kenya they found some at the southern end of the Great Rift Valley. It's a hotbed of tectonic activity where ancient layers are forced to the surface.
Ten million years ago, Africa was a much wetter place, a tropical jungle which has been slowly drying out ever since.
But these rocks in Kenya show that Africa's gradual drying trend was punctuated by bursts of wild climate fluctuation. Rick Potts is an expert in reading the rocks.
RICK POTTS (Smithsonian Institution): This layer right here represents about a thousand years of environmental stability, but then we had an abrupt volcanic eruption, and then the lake was around for perhaps 500 years, before a drought, and the lake came back. So in some cases we saw this through layer after layer of environmental change.
NARRATOR: With his trained eye, Rick could see some layers were once lake beds, others desert sands, still others came from volcanic eruptions, a snapshot of a million years of climate history.
This observation led him to an amazing new idea, rapid change as a catalyst for our evolution.
RICK POTTS:And I began to think that, well, maybe it's not the particular environment of a savannah that was important, but the tendency of the environment to change.
NARRATOR: Could it be that the need to survive violent swings of climate made our ancestors more adaptable?
A group of scientists has come here from Germany to find out just how radical these swings of climate really were.
It's hard to believe, but these huge rock formations are made of the shells of tiny one-celled organisms called diatoms.
There are many different kinds, but they all live in water. Their shells collect in layers of rock that pile up over millions of years, proving that this whole valley was once a giant lake.
Annett Junginger analyzes these rock samples under the microscope.
ANNETT JUNGINGER (University of Potsdam): What I've discovered was that those white layers consist of a special kind of diatoms, which only live in deep lakes.
NARRATOR: But between the white layers she also finds other species of diatoms which only live in shallow water.
It means that in this spot, a massive lake appeared and disappeared and reappeared many times.
JOHN KINGSTON (Emory University): These lakes are, are really significant. These are not small little ponds. And what we've been able to document now is a series of lakes that are cycling.
MARK MASLIN:When we're talking freshwater lakes the size of Lake Victoria, filling the whole Rift Valley...and then disappearing.
RICK POTTS:Enormous amount of water rushing through this area.
JOHN KINGSTON: This constant flux of turnover, of change.
RICK POTTS:An awful time to live here.
JOHN KINGSTON: It's not just a uni-directional change. It's going back and forth.
NARRATOR: Against the backdrop of a slow drying trend, Africa was periodically pulsing with climate change: wet, dry, then wet again, sometimes in the space of a thousand years.
Punishing drought alternated with storms and monsoons. Rivers and forests sprang up, then turned to dry grassland, all in the evolutionary blink of an eye.
MARK MASLIN:So we have a complete change of our ideas, from this slow drying out, to this incredible change between wet and dry, wet and dry.
NARRATOR: What effect did that have on our ancestors? Could these periods of climate instability be the key to understanding the evolutionary leap from small bipedal apes to the larger-brained toolmaker, Homo habilis?
To know that, scientists needed a detailed record that went back further than the diatoms–way back to the time when Homo habilis was evolving 2,000,000 years ago. That's only found in one place: under the ocean.
Layers of deep sea sediment tell a story that goes back millions of years. They have to be drilled from the ocean floor.
At his laboratory in upstate New York, Peter Demenocal keeps thousands of columns of sand, silt and rock: a library of ocean cores.
PETER DEMENOCAL: One of the really attractive features about ocean sediments is that they accumulate very slowly but very gradually and continuously over time.
NARRATOR: Each three-foot long core holds a continuous record of dust carried on the wind, from Africa into the ocean, where it now sits on the bottom.
An expert eye can detect distinct layers: thick in dry years when the dust is easily picked up by the wind, thin in wet years. By measuring the layers, they can tell when the climate was wet or dry.
PETER DEMENOCAL: So we can read these deep-sea sediments almost like an Earth history book of past changes in climate.
NARRATOR: To make sense of all this dirt, they have to know when it blew into the ocean. They can do this by dating the shells of tiny sea creatures that sank to the bottom at the same time.
PETER DEMENOCAL: So this gives us an age, the other analysis gives us the climate.
NARRATOR: Peter took this finely detailed climate diary and compared it to the grand arc of our human evolution. For the 3,000,000 years between Toumaï and Selam, when brain size was flat-lining, African climate was stable: dry getting a little drier. Then came 200,000 years of wildly varying climate, careening unpredictably between wet and dry.
During that time, stone tools appeared, along with the larger-brained creatures that made them. Africa was also home to many other human-like species. Climate instability put pressure on all of them.
PETER DEMENOCAL: So there are these time periods when African climate was really unstable, so anything that was living there at the time would really have to adapt to dramatically different climate changes.
NARRATOR: Those that couldn't adapt died out, like Selam and Lucy's kind. Better problem-solvers, like Homo habilis, survived.
The new discoveries about ancient climate upheavals in Africa have led Rick Potts to formulate a bold theory of human evolution.
RICK POTTS:The traditional idea we have had about human evolution is that it was the savannah, the grassy plane with some trees on it that was the driving force. But instead, what we've discovered is that climate changed all the time.
And so the idea that we've come up with is that variability itself was the driving force of human evolution, and that our ancestors were adapted to change itself.
NARRATOR: It is a simple but revolutionary idea: human evolution is nature's experiment with versatility. We're not adapted to any one environment or climate, but to many; we are creatures of climate change.
MARK MASLIN:I think we should actually look to our proud ancestry and how we evolved in East Africa and say, "That's how we survived that. We can survive the future, because we are that creature, because we are that smart."
NARRATOR: Today, climate change seems to threaten our survival, but it may have held the keys to the astonishing story of how we became who we are, because it didn't stop 2,000,000 years ago. These dramatic upheavals would continue for another million and a half years, propelling our ancestors down a road leading, ultimately, to the smartest creature the world has ever known.
End Part One