"[Lucy] was probably a generalist in terms of dietary choice, an omnivore. Although she was a biped, and successful at that, bipedalism is not a particularly apt way to get about, if avoiding predators is your goal.
"And no doubt Lucy and other members of her species fell prey to the carnivores that inhabited the community in which they lived . . .
"The question as to why Lucy stood up is a very difficult one. There are a number of competing ideas in the community as to what the advantage might have been, some of which sound credible and some of which sound incredible." (Interview, August 1993.)
Elisabeth Vrba has identified two periods of huge overall climatic change: "One of these is the time between seven and four and a half million years ago, and the second is between three and two million years ago. Both were times of dramatic cooling, major climatic changes, and of massive evolutionary changes right around the globe, both in the ocean and on land.
"And it so happens that these are the precise two time periods that have witnessed the most dramatic advances in human evolution. First, bipedalism arose between seven and about four and a half million years ago, and second, the advent of the expanded brain of the genus Homo and of stone tools, nearly two and a half million years ago." (Interview, August 1993.)
Vrba says that the evolutionary process over the last sixty million years has taken place against a background of steady cooling of the globe, and that within that cooling there have been frequent, dramatic fluctuations. A major one of these occurred between seven and five million years ago.
"The evidence is accumulating from all continents and all oceans at this moment. The oceans became about five to ten degrees cooler, the average temperature of the globe became considerably cooler. On a land mass like Africa, one of the major effects was that the warmer, wetter, more forested areas, habitats, were shrinking wholesale. Climates, in most places, became much more seasonal. Food and water would suddenly not be there, all the year round.
"Tree cover and forest cover in Africa shrunk very dramatically. Any creatures that were dependent on sheltering in the trees and on fruits and leaves from the trees as food would be dramatically affected by this.
"Suddenly these early creatures, the ancestors of which used to live in the trees, in the high-canopy forests of Africa, found themselves in much more open savannah and woodland situations. And they had to adapt or die. And I believe that bipedalism may be a direct response to this opening up of the vegetation." (Interview, August 1993.)
This argument depends not so much on the idea that getting around on two legs is better or worse than getting around on four, as that getting around on four is specifically linked to a successful life in and around the trees. When the trees retreat, or grow further apart, the specific characteristics designed for tree life (long and powerful arms, fingers and hands shaped for gripping branches) become redundant and are replaced by a different shape more suited to the new conditions.
Before the big climatic change identified by Vrba, forests spread right across Africa; now they are concentrated in the center and west of the continent, where the four-legged chimpanzees and gorillas live. As the forests retreated, Lucy and the other members of her species adapted to a two-legged life in the open.
Vrba says that she was taught in the Darwinian tradition of the inexorable, relentless process of evolution. But when she went into the laboratory at the Transvaal Museum in South Africa for the first time, she was struck by the evidence of the absence of change. The fossil record shows that for many millions of years in some cases the physical appearance -the morphology - of many creatures stays pretty much the same.
Vrba's new theory is that evolution is in fact a very conservative process, "because organisms and species are complex systems, with many interacting sub-parts. And for these systems to function, they require very particular external circumstances. From this derives the idea that unless those external circumstances change, the living systems will remain conservative."
The pacemaker of evolution would be any external circumstance which affects the habitat of a species. "The most conventional way of understanding that is climate - whether it rains more or less and whether the temperature's higher or lower. But we have to think a little more broadly to understand evolution and include all kinds of tectonic change, by which I mean, for instance, a rift valley opening, as we know happened during the time of human evolution in East Africa.
"And continents drifting, changing their positions, mountain chains building up; the sea level rising and lowering. All these physical changes together, which actually express themselves locally in terms of climatic change, would be the initiator of evolutionary change."
Species can respond in three ways to these massive changes. They can move their habitat to a new geographical location consistent with the conditions of their former living area. Or they can become extinct, if for example there is no other habitat they can move to. Or finally, on rare occasions, genetic novelty will arise and a new species will be produced.
The involvement of environmental changes
in the evolutionary process is not a new concept. Traditionally, environmental
changes were part of a package of influences including competition and pure
chance. Vrba's bold new idea is to "limit the initiation of the causal
chain entirely to physical environmental changes. They are the sine qua
non of the initiation of both speciation and extinction."
Peter Wheeler, an evolutionary biologist at Liverpool University who has
made a special study of heat loss and retention, argues for a key positive
advantage of standing upright. In hot conditions, especially when the sun
is fully up, less of the body is exposed to direct sun light, and accumulated
heat in the body can be lost more quickly without consuming large quantities
of water. Further, Wheeler thinks that two-legged locomotion, at least at
low walking speeds, is less demanding of energy than its four-legged equivalent.
This ties in with Vrba's notion of the hominids having to cope with being
in the open instead of the forest. The climate was cooling and drying enough
to cause the forests to retreat, but the sun was very bright and the early
hominids would have been directly exposed to it.
In hot, dry, open conditions, two-leggedness put the hominids at a distinct
advantage, according to Wheeler: "Of all the animals on the savannah,
we are the ones that are able to cope with the highest thermal stress .
. . We keep the whole body cool, and we are able to protect our brains in
that way." (Interview, September 1993.) And the ability to keep the
brain cool, especially as it grew and became more powerful, was an indispensable
key to human development. These explanations, from Vrba and Wheeler, emphasize
the climate and the environment.
Another, partly contradictory view comes from the anatomist Owen Lovejoy.
He thinks the riddle of upright walking cannot be solved unless a cause
is found which fits the general behavior pattern of the animal, and he detects
weaknesses in all the other arguments.
"Many times people will try to come up with a reason for walking upright
which is singular. It's been suggested that it's a feeding adaptation, to
pick fruit off trees, because chimpanzees stand bipedally in order to do
that - but the fact that chimpanzees are fully capable of doing that and
then once again return to quadrupedal walking, which is a very effective
way to locomote, tells us that's not a good selective reason to adopt upright
locomotion as a permanent adaptation." (Interview, August 1993.)
He applies similar principles to other arguments. Seeing over the long grass is not valid, because upright walking probably began in the forest, not on the savannah. According to Lovejoy, there is a similar problem with the heat dispersal argument: upright walking began in the forest, when the heat of the sun could be more easily avoided.
The idea that bipedalism began before the retreat of the forests is based on Lovejoy's estimate that by the time Lucy was alive, the species was already very well established with that mode of locomotion, and that its starting-point must have preceded the climatic change which altered the landscape.
Lovejoy believes that the strongest pressure for change comes from reproductive behavior. Anything which increases the chance that babies will survive and thrive, will lead to a major new adaptation. And he believes that upright walking enabled males to gather highprotein foods from considerable distances and carry them back to their infants; the carrying was the key.
"Any behavior that would tip the balance and make those offspring more likely to survive would be strongly selectively favored. One thing that would do that would be if males were to become secondarily involved in the parenting process.
"A way they can do that is occasionally to collect highly valued items of food, such as reptiles, amphibians, eggs, nestlings, grubs, worms. These items are extremely high in calories, in protein and in rare fatty acids and normally a hominoid must search for a long time to find them.
"If a male were to occasionally
find one of these and provide it to the female it would greatly improve
the time that she has available to care for the infant, it would raise the
survivorship of the group and of the infant, and it would be an intensively
favored form of adaptation. But that form of adaptation requires upright
walking so that you can carry items like that." (Interview, August
1993.)
And once you start walking upright you have to carry on, and get better at it. Infants, for example, lose the ability to grasp with their feet, so they have to be carried in the parents' arms rather than clinging to their fur. This in turn puts more pressure on the parents to walk consistently on two legs.
This begins to sound as though something resembling modern family units originated over three and a half million years ago, coinciding with the beginnings of bipedalism.
Social arrangements do play a major role in evolution. Much research has been done to explore how mating patterns and sexual behavior relate to reproductive advantage. In line with Lovejoy's ideas, many scientists have suggested that monogamy among humans came about because of the demands of child care, especially once larger- brained babies were being born which need so much care in their early years that the mothers need a devoted assistant.
On the other hand there is no biological reason, as Robin Dunbar points out in an article in the Cambridge Encyclopedia of Human Evolution (Cambridge University Press, 1992), why that assistant should be the child's father or indeed any other male. The assistant could just as well be the mother's sister or any other devoted friend.
And there is evidence to show that, in terms of evolutionary history, polygamy rather than monogamy has been the more usual human arrangement. The substantial difference in size between early male and female humans, and features related to sexual display in males, such as abundant body hair, are characteristics found in other species which are polygamous.
In the end any behavior which benefits reproduction and child survival will be likely to prevail. The point at which monogamy seemed to meet those needs is unclear, and it seems to stretch the imagination and the evidence too far to associate exclusive sexual partnerships with the gathering of food three and a half million years ago.
Lovejoy's scenario, describing the circumstances
which may have brought upright walking into being, is imaginative; it paints
a compelling picture of what life might have been like.
On the other hand, it is based in part on his proposal of a very early date
for the origin of bipedalism, an idea which is not widely shared.
One major strength of his argument is that no single explanation will do: the only way to begin to understand the concept of walking upright is to take account of the whole package: the climate, the changing environment, the social and family arrangements of the animals, the demands of child-rearing.
Robert Foley adds another element to the package, a reminder that evolutionary change does not happen on a global scale but locally, in response to very specific circumstances. Considering evolution from the perspective of millions of years, he says, can be very misleading.
"Looking at the relationship between climate and evolution reminds us that it is the local, not the global environment that is important in the way evolution works; and so too it is necessary to remember that while evolution may occur over geological time, its underlying mechanisms - selection and drift- are played out every day and over very short time-spans. In reality the generation, not the geological epoch, is the key unit of evolutionary time." (The Curl Lecture, November 1993.)
So the adaptation to upright walking may initially have taken place within a small community of animals in a particular clearing in the forest in response to social, feeding and reproductive pressures- against the background of climatic change rather than as a direct result of it.
The evidence from Lucy and Laetoli is only twenty years old, and in that short time, scientists have had to try to make sense of the new information that walking upright was definitely the normal way for hominids to get around from as long ago as three and a half million years - and possibly a lot longer.
And yet whatever Lucy's skill on two legs, her brain was still ape-sized. The first hominids were upright apes.