A New History of Life (50 page)

  
5.
   
The Ordovician Mass Extinction
. Wholesale extinction of tropical species. Caused by cold or perhaps by sea level change.

  
6.
   
The Devonian Mass Extinction
. Benthic and water column animals in the sea—the first greenhouse extinction?

  
7.
   
The Permian Mass Extinction
. Land and sea greenhouse extinction.

  
8.
   
The Triassic Mass Extinction
. Land and sea greenhouse extinction.

  
9.
   
The Cretaceous-Paleogene Extinction
. Combined greenhouse and impact extinction.

10.
   
The Late Pleistocene-Holocene Mass Extinction
. From 2.5 million years ago to today—climate change and human activities.

It is the last on this list that should worry us. The others, especially the greenhouse extinctions, should terrify us, but they do not, because they were—and would be—too slow moving. The slow death … and not for our species. We are pretty extinction-proof. We would
be alive, yes, but happy? On an empty planet? Surrounded by our domestic animal and plants, whose jumping genes will make their own perverse and unpredictable Cambrian explosion in the long run.

In 2010, a traveling exhibit from Ethiopia
³
brought one of the most famous of all fossils to the United States: Lucy, the early hominid.
⁴
At about three and a half feet tall, with remains that total only 40 percent of her original skeleton, in fact there is not a lot to Lucy. But she has told us a great deal.

Sexual dimorphism is the term used to describe the two different morphologies of males and females of a species. It is certainly not limited to hominids, and it is not the case that the larger of the dimorphs is always the male. In many animals, for instance, including a variety of cephalopods (excepting
Nautilus
, interestingly enough), the female morph is the larger. Apparently it takes more organ mass to produce eggs than sperm. In hominids, however, from chimps to us humans, the male is the larger. The dimorphism in humans is statistically significant, and appears to range from females being about 90 to 92 percent the height of males, depending on race. In Lucy’s kind, however, it was quite a different story.

Lucy is far from the only fossil skeleton of her kind. Her species,
Australopithecus afarensis
, is now far better known compared to our understanding (or lack thereof) when a team led by Don Johanson found her in 1974. One of the more recent finds is of a male skeleton of her kind that is complete enough to allow a good estimate of his height in life. He is called Big Man. He was five feet tall to Lucy’s three and a half. Her chin would have come just above his navel if standing face-to-face—except it had been face to lower chest.

If Lucy and Big Boy are representative of their genders in
A. afarensis
, it means that females were only 70 percent as large as their men. There had to be consequences to this—behavioral as well as cultural. In 2012, when anthropologist Patricia Kramer of the University of Washington did a detailed study
⁵
on the relative walking
speeds of males and females, based on their leg lengths, she discovered that Big Man’s optimal walking speed would have been 2.9 mph, but Lucy’s would have been a rather slower 2.3 mph. Keeping up with males would have been taxing for females—and living in a world filled with predators, being constantly in a state of anaerobic respiration would not be a very good survival tactic. Kramer thus suggested that like chimpanzees, male and female hominids spent much of the day apart, ranging separately as they foraged and hunted for food.

Other new fossil finds from Africa are also turning over some long-held views. Lucy and her kind are invariably reconstructed in dioramas or illustrations as walking upright through the Late Pliocene world of north and eastern Africa—a place with a mosaic of grassland and small patches of open forest. But for the first time ever the shoulder blades of a female of Lucy’s species—but coming from a time interval about a hundred thousand years before Lucy—show features that suggest she and her kind were tree climbers as well as adapted for walking on the ground. The question of whether these distant ancestors of ours also spent significant time in trees has been hotly debated,
⁶
largely because until this new find, there was no way to see the morphological adaptations necessary for a tree climber. The new view seems to be that australopithecines may not have come down from the trees as early as currently believed.

While hominids are new arrivals on Earth, our group, the primates, dates well back into the Cretaceous, and we have an ancestor,
Purgatorious
, that survived the K-T mass extinction itself—which is lucky for us. Some of the earliest primates belonged to the lemur branch. By 45 million years ago, more advanced primates—the first true anthropoids, which today include monkeys, apes, and humans—appear in the fossil record of Asia. The oldest of these was found in China and is now named
Eosimias
.

About 34 million years ago, surely smarter, definitely bigger, and perhaps more aggressive monkeys evolved. One of these, named
Catopithecus
, has a skull the size of a small monkey’s, a relatively flat face, and is the first primate to sport the same arrangement of teeth humans have—two incisors, one canine, two premolars, and three
molars. We now have a good idea of our own evolutionary tree, right up to where and when “humans” can be said to first appear—the African genesis of the australopithecines.

Paleoanthropologists have done a remarkable job of deciphering the where and when of the speciation event that produced our species. The human family, called the Hominidae, seems to begin as much as 5–6 million years ago with the appearance of Lucy and her kind, the
Australopithecus afarensis
described above. Since then, our family has had as many as nine species, although there is ongoing debate about this number, which seems to change years as both new discoveries and new interpretations of past bones make their way into print. But the most important descendant of the early pre-Pleistocene hominids is the first member of our genus,
Homo
, a species named
Homo habilis
(handyman) for its ability to use tools, which is about 2.5 million years old. This creature gave rise to
Homo erectu
s about 1.5 million years ago, and
H. erectus
either gave rise to our species,
Homo sapiens
, directly about 200,000 years ago, or through an evolutionary intermediate known as
Homo heidelbergensis
. Our species has been further subdivided into a number of separate varieties. Some workers consider the Neanderthals to be a variety, while others interpret it as separate species,
Homo neanderthalensis
. A great deal of new work on recovered and decoded Neanderthal DNA
⁷
is one of the most intriguing aspects of human paleobiology, with the latest evidence suggesting that the human and Neanderthal lineages diverged before the emergence of contemporary humans and our current DNA. They did not come from us, nor did we come from them. We both evolved from a common extinct ancestor different from both species.
⁸

Each formation of new human species occurred when a small group of hominids somehow became separated from a larger population for many generations. In the 1960s and 1970s there was a view that modern humans came about from what has been called a candelabra pattern of evolution—that all over the planet separate stocks of archaic hominids such as
Homo erectus
all evolved into
Homo sapiens
at different times and places. This notion now seems laughable.

The fossil record tells us that the so far oldest member of our species—variably called a modern to distinguish it from more archaic forms of
Homo sapiens
—lived 195,000 years ago in what is now Ethiopia. It is unknown and not terribly important whether this fossil represents the oldest tribe of us or was from a group that wandered in from the true origin place and was fortuitously fossilized in Ethiopia. But very soon after, this band set out walking to the farthest southern regions of the African continent, and then to the north as well, finding a way out of Africa through Eurasia—and in so doing they spread out across the globe,
⁹
effectively isolating themselves from others of our species, and thus adapting to the very different environmental concision in which these wanderers found themselves. Quite different adaptations, morphological to physiological, were necessary for survival in the sun-starved, ice-covered north than on the plains of Africa, as well as all areas in between. As our numbers grew, so too did our variation—and our various evolutionary changes. But all of this was within the same species.

Climatologists have long theorized that climate change observed over the past two and a half million years—the alteration between long periods of very cold climate with growing ice sheets and dropping sea level alternating with shorter times of warmth—were the result of the orbital changes described above as having been first articulated by Milutin Milankovic′. Until the ice cores became available, with their unprecedented resolution in discerning climate through recent time, the changes were thought to have been slow. But with that resolution a newer view became apparent.

The ice core records and other sources of climate information such as deep-sea paleontological and isotopic records indicate that over the past eight hundred thousand years the interglacial periods—the warmer times between the much cooler glacial intervals—have lasted on average about eleven thousand years. That’s almost half the Earth’s precessional cycle, orbital changes occurring every twenty-two
thousand years. The current interglacial has already lasted more than eleven thousand years and some records suggest that we have been in the warm period for as long as fourteen thousand years. Does this mean that the glaciers are advancing at this moment? The answer to that question is a decided no, for several reasons. First of all, precession is not the only orbital aspect that affects climate. Records show that between 450,000 to 350,000 years ago there was an interglacial stage that lasted much longer than eleven thousand years. This interglacial was coincident with a time when orbital eccentricity was at a minimum. Just such a pattern of minimal orbital eccentricity is under way at this time, suggesting that the present interglacial could continue for thousands or perhaps a few tens of thousands of years into the future—or it could end at any time.

The Pleistocene epoch signaled a significant kind of climate change beginning about 2.5 million years ago. The large cool grasslands and tundra of the high latitudes during the last pre-ice-age epoch of the Cenozoic era gave way to a new kind of land cover—ice. Year by year a slow excess of snow and ice caused the formations of glaciers, which slowly crawled southward. Eventually continental glaciers began to coalesce and merge with mountain glaciers, uniting in unholy matrimony to grip the land in glacial ice and glacial winter.

By no means was the entire planet gripped in ice, as seems to be popularly imagined. There were still tropics and coral reefs and warm sunny climes pleasant the year around. But probably no place on Earth was unaffected in at least some minor way; the global climate changed, causing shifts in wind and rain patterns. Even those places far from the ice were climatically changed, perhaps colder or even warmer, often quite dryer. Gigantic cold deserts and semideserts expanded in front of the advancing ice sheets, while regions normally dry, such as the Sahara desert of northern Africa, experienced increased rainfall. Conversely, the great rainforests covering the Amazon basin and equatorial Africa, regions of relative climatic stability for tens of millions of years prior to the onset of the ice age, experienced a pronounced cooling and drying such that large tracts of jungle retreated into pockets of forest surrounded by wider regions of dryer savannas.

Many of these rapid climate changes occurred while humanity was colonizing the globe. By about thirty-five thousand years ago, it appears that the final evolutionary tweaks had occurred, making us as we are now. We can call these new humans the moderns, and they conquered the world bit by bit. They arrived in each new region slowly yet inexorably. It didn’t happen in a century. It didn’t parallel the taming of North America by Europeans, when several centuries saw the transformation of a giant native-vegetation-covered continent to a giant agriculture- and concrete-covered continent. It was instead a slow conquest, with millennia falling away like leaves as the moderns slowly spread over the globe. Even the island continent of Australia had become the habitat of
Homo sapiens
thirty-five thousand years ago. Northern Asia, however, was still undiscovered. And beyond Asia, an even bigger territory, North and South America, had still not experienced the first human footfall.

The first people to arrive in the vast tract of what is now Siberia were Paleolithic big game hunters. They arrived as long as thirty thousand years ago, with a tradition already in place for existing in this harsh climate. Eastern Siberian stone tools show some differences from the European traditions of the time, and were clearly influenced by the flake cultures of Southeast Asia. Yet the major technology, the construction of large spearpoints, was formulated for killing large animals.

The arrival of the first humans in Siberia was set against a time of slight warming, and this warmer period, following a cooler time, may have encouraged the spread of humans into an otherwise hostile region. Yet soon after their arrival in Siberia the Earth began to cool again, and by twenty-five thousand years ago a major glacial event was well under way.

In western Europe and North America the great continental ice sheets were inexorably spreading downward to cover vast regions with ice a mile thick. In Siberia, however, there was so little moisture that the ice was unable to form. Into this vast treeless frozen territory,
humans expanded ever eastward. Because there was so little wood, the hides and antlers of their prey became important resources, and the very bones of the largest quarry—the mastodons and mammoths—were used for housing. These people became—by necessity—big game hunters, and their principal prey may have been the mammoth and mastodon.