Masters of the Planet (26 page)

Early geologists constructed a chronology of the Pleistocene using physical evidence of the advances and retreats of the glaciers—such as horizontal scratches on valley sides and floors made by pieces of rock carried along in the ice, or the deposits of such rock that were dumped when the glaciers melted. But the problem was that each glacial advance scoured away much of the evidence left behind by its predecessors, and the resulting observations were a nightmare to interpret. Since the 1950s the older division of Pleistocene time into four major glacial and interglacial periods has thus given way to a chronology based on modern geochronological and geochemical analyses of long cores drilled through sea-floor sediments, or through the Greenland or Antarctic ice caps.

In both cases, the favored approach has been to measure the ratios of lighter and heavier isotopes of oxygen in the layers of the accumulating ice itself, or in the shells of microorganisms that lived in the surface waters and sank to the seafloor when they died, forming a sediment pile. This ratio provides a guide to prevailing temperatures because the lighter isotope more readily evaporates from seawater than the heavier one does. When the vapor is precipitated as rain or snow over the poles
in
cold times, the light isotope becomes “locked up” in glacier ice. As a result, cold oceans and the microorganisms living in them are enriched in the heavier isotope, while the lighter one is more abundant in the ice caps. And the ratio between the two in an ice or seafloor core correlates closely with the temperatures prevailing when the ice/sediments were formed. These cores provide a continuous record of fluctuations in this ratio, and thus of shifts in prevailing temperatures over time.

The oxygen isotope record of changing global temperatures for the past 900 thousand years, based on
16
O/
18
O ratios in cores from the Indian and Pacific ocean seabeds. Even-numbered stages were relatively cool, odd-numbered ones relatively warm. Within each major stage there were considerable oscillations in temperature. Data from Shackleton and Hall (1989); chart by Jennifer Steffey.

From
such data, paleoclimatologists have been able to identify 102 separate “Marine Isotope Stages” (MIS) since the start of the Pleistocene, and have numbered them, starting with the most recent. As a result, warm stages are given odd numbers and cold periods get even ones. We are now in warm MIS 1, the peak of the last glacial episode is represented by MIS 2, and so forth. Within each major episode of temperature fluctuation there are numerous minor peaks and troughs called “stages,” some of them significant enough to have their own designations. Stage 5, for example, is subdivided into Stages 5a, b, c, d, and e, the oldest of them (5e) so warm that sea levels were several meters higher than at present.

Far back in the early Pleistocene, temperature oscillations were frequent though not very pronounced; but as we approach the present they have become more widely spaced, and more intense. Tying in particular hominid fossil sites to the marine or ice cap sequences is not always easy when absolute dates are not available; but since erratic environmental conditions usually result in frequent faunal changes, the identity of associated animal remains will often provide valuable clues. In any event, in combination with new methods of dating and independent measures of climate such as the analysis of fossil pollen and soils, we now have a pretty good notion of the tapestry of environmental challenges with which our precursors had to cope.

THE FIRST EUROPEANS

It is against an unsettled climatic and geographic background that we have to consider the early hominid occupation of Europe. Until not long ago, it was believed that early hominids first entered Europe relatively recently, certainly much more recently than the parts of southern Asia that hominid populations could reach by expanding along subtropical coastlines. The Dmanisi discoveries, right on the crux between Asia and Europe, showed that contrary to expectation the temperate zone had been penetrated very early on; and now there is direct physical evidence from an Iberian site that hominids had established themselves in western Europe by 1.2 million years ago. This evidence comes from a site known as the Sima del Elefante in the limestone Atapuerca Hills of northern
Spain,
and it consists of a piece of lower jaw of the genus
Homo,
bearing a few worn teeth, that is too incomplete to be assigned to any particular species. Associated with this specimen are some mammal fossils suggesting that the hominid had lived during a relatively warm stage; and stone tools of Oldowan aspect demonstrate that it was not an advance in technology that had permitted hominids to penetrate the Iberian peninsula at this early date. As far as lifestyle is concerned, there's nothing much beyond this to go on. More as a matter of convenience than anything else, the scientists who discovered the Sima del Elefante fossil tentatively associated it with some similarly fragmentary hominid fossils from the nearby Atapuerca site of the Gran Dolina, that they had previously assigned to the new species
Homo antecessor.

This latter species, some 780 thousand years old, is of particular interest because the Atapuerca scientists believe it may represent the common ancestor of the lineages that led to
Homo neanderthalensis
on the one hand, and to
Homo sapiens
on the other. But while the Gran Dolina does lie more or less in the right time zone for its hominids to play that role, just where they stand in the evolutionary picture remains equivocal. Indeed, it seems at least equally likely that the
Homo antecessor
fossils are evidence of an early “failed” hominid foray out of Africa and into Europe that did not have a direct ancestral connection to the Neanderthals who later established themselves in the European Peninsula. Still, if there is a direct connection back to the Sima del Elefante hominid, the first hominid occupation of Europe was a long one; and if you are looking for continuity, you might find further evidence for it in the fact that the crude stone tools from the two Atapuerca sites do not differ much.

The spot at the Gran Dolina where the hominids were found seems to represent an ancient cave entrance that was occupied by hominids during a relatively mild and humid period. And the Atapuerca scientists' claim that
Homo antecessor
represents the common ancestor of Neanderthals and modern humans is no less remarkable than their conclusion that the bone fragments assigned to this species show evidence of cannibalism. Fossil bones so far recovered from the Gran Dolina are typically badly broken, and many of them bear marks left by slicing, chopping, and scraping with stone tools, along with fractures of a kind strongly suggesting butchery. What's more, allowing for differences in
anatomy
among species, all of the bones, human and nonhuman alike, were treated in an identical fashion. And this implies that all of the cadavers they represent were used for the same purpose, namely consumption. There is certainly no evidence of any special or ritual treatment of the human remains. As a result, there is a strong case to be made that hominids were eating other hominids at the Gran Dolina 780 thousand years ago; and though not everyone is entirely happy with this interpretation, the argument for cannibalism is a strong one and the Atapuerca team has recently issued a robust defense of its conclusion.

Demonstrating cannibalism is just the beginning, as it raises a host of questions, foremost among them, who was eating whom, and why? To modern humans, cannibalism has all kinds of symbolic overtones, depending, for instance, on whether you are eating your kin or strangers. The Atapuerca group dismisses any symbolic implications at the Gran Dolina, emphasizing that the 11 children and adolescents represented in the butchered sample received no special handling, and that the butchery techniques involved were designed to extract the maximum amount of edible material, including the brains. Because they can find no evidence to support the idea that the butchered humans at the Gran Dolina were the victims of a single episode of group starvation— indeed, the butchery may have taken place over several tens of thousands of years, in the midst of a rich habitat—they propose that cannibalism was a regular part of the subsistence strategy of
Homo antecessor.
They even go so far as to speculate that the tender ages of those eaten might imply that they were the vulnerable victims of hunters sent out to raid neighboring groups.

Sadly, there is not much direct evidence from the Gran Dolina itself, apart from the butchered fossil carcasses and the stone implements, most of them knapped right there in the cave, that were used to dismember them. There is no indication of the use of fire, for example, or of any other activities that might have been associated with the hominid occupation—although some plant vestiges might indicate a more rounded diet than the bones alone suggest. Almost certainly, the Atapuerca researchers are right to reject any implications of ritual in the butchered hominid assemblage; and if they are correct in concluding that cannibalism of this very matter-of-fact sort was a routine component of dietary life at the
time,
the clear implication is that the hominids concerned did not have the kind of regard for others that is typical in modern human societies today. Wherever it has been documented in the historical record, socially sanctioned cannibalism, whether within or among groups, has always been a “special” activity, surrounded by its own rites and ambivalences. The chillingly prosaic nature of the cannibalism at the Gran Dolina implies something totally different—and to us, totally alien.

NEANDERTHAL ORIGINS

Although it is not possible to draw a straight line connecting
Homo antecessor
at the Gran Dolina with the later
Homo neanderthalensis,
yet another site in the astonishingly fecund Atapuerca region furnishes us with the best evidence we have for an early member of the Neanderthal lineage. The species
Homo neanderthalensis
itself does not show up in the European fossil record until less than 200 thousand years ago; but a stone's throw away from the Gran Dolina, the large Cueva Mayor cave has produced one of the most extraordinary phenomena in paleoanthropology, and one that gives us a marvelous insight into the early stages of the Neanderthal lineage. Well within the cave is a vertical shaft, almost fifty feet deep, at the bottom of which a small, cramped chamber has delivered the greatest concentration of hominid fossils ever discovered, anywhere. Hominid fossils are an extraordinary rarity, and paleontologists usually count themselves lucky to find just one or two. But the lead excavator of this Spanish cornucopia once remarked to me that his team was the only one in the world with the luxury of deciding how many hominid fossils it wanted to excavate in the next field season—a few dozen, a hundred—and then quit when the quota was filled. No wonder this amazing place is called the “Sima de los Huesos”—the Pit of the Bones. It's a hellishly cramped, difficult, and uncomfortable place to excavate, to be sure; but well worth every painful moment.

The Sima was initially discovered by spelunkers, who alerted paleontologists when they discovered the bones of extinct cave bears. And since systematic excavations started there in the early 1990s, the site has produced hundreds of hominid fossils, representing the remains of at least 28 individuals of both sexes. Although the individual bones are
typically
badly broken, the preservation of the bone itself is remarkable, and scientists have been able to reassemble the many hundreds of pieces into half a dozen more or less complete crania and numerous elements of the postcranial skeleton. A homogeneous sample this large of a single extinct hominid species from one place is unprecedented; and the Sima gives us a unique glimpse into the biology and even the demography of an extinct hominid species. The individuals whose bones were found at the bottom of the pit range from a single child to a handful of older adults in the 35- to 40-year range, and half of them had died between the ages of 10 and 18. Presumed males were larger than presumed females to about the same degree you find in modern humans, and one male stood almost six feet tall.

These were heavily built folk, with robust bones, and each probably weighed a good bit more than a modern human of the same height. Almost certainly, they were immensely strong compared to us. At the same time, their brains were on average a little smaller than ours, three crania ranging in volume from 1,125 cc to 1,390 cc. Their pelvises were broad, with birth canals capable of accepting the head of a modern newborn. Once individuals had emerged into the world, they faced a life reasonably free of dietary stress. Episodes of malnutrition are reflected in the enamel of developing tooth crowns, and evidence of such events is rarer at the Sima than it typically is in recent
Homo sapiens
populations. This is something not unexpected in an environment that is believed to have been rich and productive.

Studies of the mammal bones found at sites more or less contemporaneous with the Sima imply that northern Spain had cooled off a bit since Gran Dolina times, but that the Sima folk had lived in an open woodland landscape supporting a diverse fauna. The Atapuerca researchers believe that the Sima hominids would have been major predators in this setting, though they would also have been in competition with at least two species of lionlike large cats, recent arrivals in the region. The prevalence of arthritis in the Sima individuals' jaw joints coincides well with heavy wear on their teeth that indicates not only that they ate a fairly tough and abrasive diet, probably with a gritty plant component, but that they used their teeth extensively for such tasks as processing hides. And although the best-preserved skull shows
evidence
of a dental infection that may have (very painfully) killed its owner, many teeth in the sample show a concern for dental hygiene in the kind of grooving that comes with the frequent use of toothpicks.