2 pages essay geography

They find that a major cold, dry spell 2.8 million years ago forced woodlands in sub-Saharan Africa to yield to grasslands. Animal species, including prehumans, would have had to undergo major adaptations to accommodate to this shift in their environment. It is known that around this time, the ancient prehuman line apparently split into at least two branches. One was the genus Homo, which led in time to modern humans. The other was a kind of ape-human, known as Australopithecus robustus, whose line ultimately became extinct. A second dry, cool period occurring about a million years ago coincided roughly with the emergence of Homo erectus, the immediate ancestor of Homo sapiens, as the sole representative of the hominid line and its expansion out of Africa to the rest of the world.

The climatic hypothesis “seems plausible” and “the quality of the environmental record is clearly improving; it’s great,” he said. But “to me,” he said, “the great step forward will come in a dramatic improvement in the fossil record, not the climate record.”

Dr. Frank Brown, a geologist at the University of Utah who works with the ashes, sediments and fossils in east Africa, says the deMenocal work “might give us the most precise correlations in time between the hominid fossils on land and the climatic record of the deep sea; if we can pin those, we’re in good shape.”

Full Text

FROM the sea floors around Africa, scientists have found intriguing evidence about forces that may have shaped the evolution of humankind.

Cores drilled out of the sea bottom contain layers of dust blown off the surface soils of Africa. Analyzing these layers of dust back to five million years ago, a critical period in hominid evolution, paleoclimatologists have constructed a picture of changes in climate and vegetation, matching them to major developments in the evolution of humanity’s ancestors as shown by the fossil record.

They find that a major cold, dry spell 2.8 million years ago forced woodlands in sub-Saharan Africa to yield to grasslands. Animal species, including prehumans, would have had to undergo major adaptations to accommodate to this shift in their environment. It is known that around this time, the ancient prehuman line apparently split into at least two branches. One was the genus Homo, which led in time to modern humans. The other was a kind of ape-human, known as Australopithecus robustus, whose line ultimately became extinct. A second dry, cool period occurring about a million years ago coincided roughly with the emergence of Homo erectus, the immediate ancestor of Homo sapiens, as the sole representative of the hominid line and its expansion out of Africa to the rest of the world.

The relationship between climate and evolutionary change has been suggested before, but the new study gives much more detailed evidence of vegetational changes in Africa. It ties these changes to a series of ice ages in the Northern Hemisphere, and also provides an invaluable new series of time markers in the form of layers of volcanic ash buried in the ocean sediments. Since the same ash is found on land associated with several fossil finds, the dating of climate change and human evolution can be linked far more tightly than ever before.

“We can now place some of the fossils within a specific wiggle in the climate record plus or minus 10,000 or 20,000 years, which is very fine resolution,” said Dr. Peter deMenocal, a paleoclimatologist at the Lamont-Doherty Earth Observatory of Columbia University, who reported the findings at a meeting of the American Geophysical Union last week in San Francisco.

The match-up has already been made for the period from 3.5 million to 4 million years ago, but has not yet been applied to the critical later periods when the human family tree branched. That, said Dr. deMenocal, is the next step.

Through such means, a picture of great ecological sweep and drama is slowly coming into clearer focus.

According to the climatic hypothesis of human emergence, whose chief proponent is Dr. Elisabeth S. Vrba, a paleontologist and evolutionist at Yale University, climate affected the human emergence in this way:

Near the end of the Miocene epoch, some time from five million to six million years ago, a cooling and drying of the global climate caused African grasslands to expand and rain forests to contract. At least one species of tree-dwelling ape left its shrinking forested habitat and learned to forage afar on the savanna, a grassland dotted with stands of trees. In the interest of efficiency, the creature began to move around on two legs and eventually evolved into the first hominid, or human-like creature, called Australopithecus.

Between the late Miocene cooling and about three million years ago, the climate in most of tropical Africa fluctuated from mildly warm-moist and mildly cool-dry states. Savanna woodlands proliferated in warm-moist times, but shrank and became fragmented when the climate changed, the theory holds. Some local populations of Australopithecines became isolated within the fragments and some lived on the surrounding grasslands. If they had remained separated long enough, they might have evolved in different directions, but they came into contact again after the climate shifted and the woodlands re-expanded.

Whether for that or other reasons, two separate lines of hominids emerged during the major cooling and drying that set in about 2.8 million years ago. At the same time, many woodland species found themselves permanently deprived of suitable habitat by a great expansion of grasslands. Populations of plant and animal species shrank, and many species became extinct. Competition for food intensified the pressure. Individuals that squeaked through and adapted to the new habitats became the nuclei of new species, which eventually radiated outward through the transformed environment. Two Branches Emerge

Within a few thousand years after the cooling, at least one group of hominids to emerge from this evolutionary “pulse” had evolved a mainly vegetarian existence in denser patches of savanna vegetation along rivers. This group probably grubbed for coarse, hard seeds and tubers in the driest seasons when food was scarce. Paleontologists call these creatures robust australopithecines.

Members of a second, more lissome and adventurous group roamed far and wide, making themselves at home in many types of habitats and exploiting many kinds of foods, in time including meat. Meat improved nutrition, since it was available year-round. Some evolutionists believe that the need to master a wider range of resources would have put evolutionary pressure on this group of hominids to become more intelligent, thereby promoting the development of more powerful brains. Paleontologists have labeled these creatures the genus Homo.

The robust line may have lost out in competition with Homo, but Dr. Vrba believes it is more likely that the intensified cooling and drying about a million years ago caused the robust line’s extinction. In any case, by about one million years ago, Homo erectus, the immediate progenitor of Neanderthals and modern humans, was poised to move out of Africa, colonize the world and evolve into Homo sapiens.

Dr. Vrba suggests that the way might have been cleared by the intensified cooling, which, according to Dr. deMenocal, was caused by a Northern Hemisphere ice age. Ice masses lock up water and sea levels fall, opening up better land connections between Africa and central Asia.

Dr. deMenocal analyzed ocean-bottom corings from four Atlantic sites off the west African coast and one site each in the Arabian Sea and the Gulf of Aden off east Africa. He identified layers of soil and other particles blown off the continent by winds and buried in the nearby deep-sea sediments over millions of years. Dust layers were thickest when the African climate was driest.

Also showing up in the layers of sediment were characteristic particles of silicate that help grasses stand up straight. These remain when the grasses themselves decay and they, too, are blown into the ocean. More of them in the sediments reflects an increase in grasslands. Role of Ice Sheets

In his cores Dr. deMenocal found thicker layers of dust and silicate particles that built up 2.8 million years ago and one million years ago, indicating drier conditions and the spread of grasslands at these times. The first dry period coincided with the first appearance of large North American and European ice sheets.

Dr. deMenocal postulates that the climate regimes of the Northern and Southern Hemispheres were independent of each other until then, but that once the ice sheets built up they dominated the climate of both hemispheres.

Based on computer simulations of the ancient climate, he believes that glaciation caused North Atlantic sea surface temperatures to drop by as much as 25 degrees Fahrenheit and that the temperature dropped by 3.5 degrees to 10 degrees over west Africa. The greater contrast in temperature between the cold Atlantic surface waters and the warmer African continent intensified trade winds, he says, blowing cooler and drier air from Europe over Africa.

The computer models also showed that as the ice sheets grew higher, they apparently diverted colder, drier air toward east Africa, causing temperatures there to drop by 3.5 to 7 degrees. By comparison, the global temperature was 5 to 9 degrees colder in the depths of the last ice age than it is now.

Many unanswered questions remain. For one thing, says Dr. Vrba, there appears to have been another pulse of evolution and a turnover of species in tropical Africa about 1.8 million years ago. Some paleontologists suggest that both the Homo and the robust lines of hominids underwent further splitting at this time. Dr. deMenocal says he has detected a period of cooling and drying around that period, but has not yet tied it firmly to the record of human evolution.

And while the deMenocal study is “a good piece of work,” none of the evidence of coincidence between climatic events and evolutionary events adds up to conclusive proof, says Dr. David Pilbeam, a paleontologist at Harvard University.

The climatic hypothesis “seems plausible” and “the quality of the environmental record is clearly improving; it’s great,” he said. But “to me,” he said, “the great step forward will come in a dramatic improvement in the fossil record, not the climate record.”

For instance, he said, there is not enough fossil evidence to determine for sure whether the Homo and robust lineages diverged about 2.6 million years ago, as many experts believe, or much earlier.

Dr. deMenocal nevertheless believes that the ability to compare volcanic ash deposits buried alongside hominid fossils with ash deposits in ocean sediments makes it possible to establish a firm link between the fossil record and the climate record for the first time. Matching Fossils With Climate

In examining corings from a sea-floor site in the Gulf of Aden less than 45 miles off the coast of east Africa, he discovered deposits of volcanic ash blown northeastward by monsoon winds from the Rift Valley of nearby east Africa, where the richest lode of hominid fossils has been found. The ashes were also deposited with the remains of the fossils. By comparing the dates of ashes in sea-floor sediments with those buried with fossils, it has become possible to match the fossil record more closely to climatic fluctuations.

Dr. Frank Brown, a geologist at the University of Utah who works with the ashes, sediments and fossils in east Africa, says the deMenocal work “might give us the most precise correlations in time between the hominid fossils on land and the climatic record of the deep sea; if we can pin those, we’re in good shape.”

Dr. Pilbeam says the fossil record may never be good enough to enable scientists to determine whether climate-driven habitat change, competition between species or some biological factor caused splitting in the human family tree.

But, Dr. Brown says, the fossil record has got better, improves every time an investigator goes into the field and “can get better yet.

Photograph

Dr. Elisabeth S. Vrba of Yale University is the chief proponent of the climatic hypothesis of human emergence. (Tim White); Dr. Peter deMenocal, left, paleoclimatologist at Lamont-Doherty Earth Observatory, and Dr. Frank Brown from the University of Utah seeking remains of early hominids near Lake Turkana in East Africa. (Ian MacDougall)(pg. C18)