IMAGINE brooding over a million-piece jigsaw puzzle with gaping holes in it, many sections in places where they make no sense, and not the dimmest foregleam of the complete picture, and you have the recorded history of human evolution. Pretty pitiable.

Faced with the awesome odds of ever exhuming one of the missing pieces and slipping it into place, scientists have little choice but to paper up the gaps with figments of imagination or extrapolation -- often wildly fanciful or coloured by personal bias and beliefs -- and then dig with a prayer on their lips. As anthropologist Leslie C Aiello of the University College of London, says, "In human palaeontology, there are often more opinions and interpretations than there are fossils."

Predictably enough, these reconstructions often fly in the face of exhumed evidence. But now, thanks to 2 major discoveries this year, our invaluable ferrets into the human past have been forced to rewrite and annotate at least 2 chapters of the record.

Over the past 2 year,fossil-hunters dug up in Ethiopia the remains of what many scientists believe to be the earliest ancestor of humans: the 4.4 million-year-old fossils haul the lineage back by about 800,000 years, close to the time when simians and early humans went their separate ways. Apart from heating up the debate over the forking off of evolutionary divorce, the find has raised questions on theories about what compelled the earliest humans to adopt an upright posture in the first place, the fundamental trait that distinguished early humans from apes (see box: Two legs to stand on).
Out of Africa? The 2nd event, in May this year, is not a new fossil find but an advanced dating method which reveals that a pair of Homo erectus fossils (our immediate ancestor) from Java are much older than earlier believed. Their new age, which makes them contemporaries of their African cousins, challenges the popular theory which holds that H erectus evolved in Africa and then spread to Eurasia.

The finding has also sparked a fresh debate over 2 important issues -- first, the role of primitive technology in making H erectus an explorer; and, second, the emergence of modern humans, Homo sapiens. The question is whether H sapiens evolved only in Africa and then scattered to other parts of the world or whether they evolved at different times in different regions.

Somewhere between 5 and 10 million years ago, global cooling transformed the dense, forested landscape of Africa into open grassland. It has long been presumed that a particularly enterprising species of ape abandoned life on trees for a life on level ground. Tramping on terra firma triggered a host of evolutionary changes. Gradually, this protohuman freed its hands from the business of propping it up trunk upwards, and stood up and waddled on 2 legs -- presumably the 1st evolutionary step towards human-ness as we know it today.

After Charles Darwin, only the most dedicated of the world's many religious orders would doubt human descent from -- awful as it sounds -- apes. Anthropologists, rooting back and forth in the debris of protohistory, have passionately debated over where, when and why the branch that led to humans peeled away from the branch that led to our closest cousin, the chimpanzee.

Till 1924, when South African palaeontologist Raymomd Dart first described the Taung skull found in a cave in South Africa, Asia was widely believed to be the birthplace of the first member of the human race. Dart's find was the bedrock of the out-of-Africa theory.

In the furious debate about why a species went 2-legged, there are 2 protagonists -- the palaeo-anthropologists and the biochemists. While molecular evidence reveals that apes and humans segregated between 6 to 4 million years ago, till a decade ago most fossil-hunters argued that the event occurred much earlier, about 20 to 15 million years ago (see box: Lineage lingers in molecular time). But in 1982, fossil-hunters and molecular biologists reached a working compromise based on new evidence: most today would agree that the separation occurred about 7.5 million years ago.
Lucy's grandpa However, the debate has erupted once again with the recent discovery in Ethiopia of fossil remains of the earliest of all human granddaddies (or grandnannies, if you will) unearthed so far. Found about 140 miles northeast of the Ethiopian capital, Addis Ababa, the 17 fossils date back more than a million years before the 3.2 million-year-old Australopithecus afarensis, coyly nicknamed "Lucy", till now the oldest superstar in the dramatis personae of human evolution. Scientists believe that Lucy was the first creature to walk upright.

The recently disinterred 4.4 million-year-old fossils of a creature named Australopithecus ramidus (ramid means root in the local Afar language) pushes back human lineage almost touching an epoch that molecular biologists believe humans and apes said goodbye to a common ancestor. The fossils were found by an international expedition led by palaeontologist Tim D White of the University of California at Berkeley.

Many experts believe that these fossils may well be the long-sought after Holy Grail of the fossil-hunters -- the mysterious "missing link" between the apes and the first human. Says British palaeontologist Bernard Wood of the University of Liverpool, "It looks to me like this is either the common ancestor or damned close to it. I think we're splitting hair not to call it the 'missing link'."

It is still uncertain whether these creatures -- about the size of chimpanzees -- were able to walk straight. The most important clues to a standing posture are the pelvic, knee and foot bones. But all that exists of these fossils are teeth, jaws, a small piece of the skull and some arm bones.

However, most experts assume that A ramidus walked on 2 legs, as did Lucy. One clue is a tiny fragment of the foramen magnum, the opening at the base of the skull where the spinal cord meets the brain. Its location suggests an upright stance. Moreover, the structure of the arm bones are different from those of knuckle-walking apes.

However, if A ramidus really walked upright, anthropologists will have to revise the lay of the evolutionary road that led to modern humans. It is already clear from Lucy, who stood upright but had an ape-like skull, that bipedalism came before the large brain that humans are endowed with. But what triggered the shift to 2-leggedness?

This question has attracted an insistent buzz of debate. The prevalent theory is that a change in climate transformed the eastern and southern African forests into dry, open grassland, favouring apes that could walk upright -- they would have been able to see predators from farther and walk long distances holding food or children.

But it appears that A ramidus did not live in the savannah but in some sort of forest, as suggested by fossils of wood, tree seeds, monkeys and bats found under the volcanic ash where the fossils lay buried. If the theory that A ramidus was a forest-dweller is supported by more fieldwork and analysis, theorists will have to form a new explanation for the development of the erect posture. Anthropologist Kevin Hunt from the US's Indiana University already has a theory explaining the 2-leggedness even in forests: he believes that forest-dwelling A ramidus perhaps walked straight to be able to pick fruit from trees in not-so-dense forests.

Bone of contention
The second bone of contention marks another crucial turning point in the evolutionary history of Homo sapiens. It belongs to our immediate ancestor, Homo erectus. Although H erectus fossils were exhumed many years ago, they have been re-dated to be at least a million years older than believed earlier.

The new dates, which make the Asian H erectus contemporaries of their African cousins, have ignited several controversies. If the new dates are correct, anthropologists will have to rethink at least 3 notions: first, the physique, personality and behaviour of H erectus; second, the role of ancient technology in triggering the spread of H erectus to Eurasia; and lastly, how and when Homo sapiens evolved.

The oldest of the H erectus specimens, a whole specimen disinterred by Richard Leakey at Koobi Fora in Kenya, was 1.8 million years old. So everything suggested that H erectus evolved in Africa about 2 million years ago, stayed put for 800,000 years and then meandered into Asia. But why didn't H erectus move out earlier? It had all the necessary ingredients of an explorer -- unlike its predecessors, it was omnivorous, and much bigger, which helped it tolerate water loss and store food and water for longer periods.

Walker and Pat Shipman of the Johns Hopkins University, Baltimore, USA, had pointed out in 1989 that the transition from a mainly herbivorous diet to an omnivorous one meant that the creature had to expand its hunting horizons to increase its food resources. Others, like palaeo-anthropologist Milford Wolpoff of the University of Michigan, USA, argue that social and dietary changes allowed the early landfarer to carry food over long distances and move through hostile terrain.

Technology takes its time
To account for the belated adventuring, scientists argued that it didn't have the necessary technological sophistication to cope with new environments. The earliest tools appear about 2.5 million years ago, but they were not good enough -- no more than pebbles and sharp flakes of stone. Not until 1.4 million years ago did anything more advanced come to hand.

The new dates from Java have demolished that theory and suggest that H erectus was sufficiently equipped anatomically and behaviourally to venture beyond the confines of the traditional cautious hominid. Some have calculated that even with a modest territorial expansion of 10 km per generation, H erectus could have covered the distance from east Africa to Java in 25,000 years.

Despite the new dates, most scientists still go by the out-of-Africa theory. But the recent findings have emboldened some anthropologists to suggest a radical alternative -- that H erectus evolved in Asia and then moved to Africa. There is, however, a problem with this idea -- as yet there are no known antecedents of H erectus, such as Homo habilis, in Asia.

But did the Asian H erectus evolve into H sapiens? Although the peripatetic H erectus may have travelled the world earlier than previously believed, a majority of scientists believe that it eventually kicked the bucket in Asia. And modern humans -- Homo sapiens -- evolved from H erectus in Africa and then colonised the rest of the world. It is argued that if the African and Asian H erectus were separated for almost 2 million years, it is virtually impossible that they both evolved into the same species.

Unlikely ripple
Also, the multiregional hypothesis entails a genetic continuity, or gene flow, throughout all Old World populations of H erectus, so that evolutionary change in one region is reflected in other regions. Many geneticists are sceptical about a ripple effect over such vast distances, let alone over a million autonomous years. But if H erectus arrived in Asia 2 million years ago, then all subsequent H erectus populations should have evolved into H sapiens from a common African stock, separated by 2 million years.

A recent analysis, however, of a fossil from China lends support to the multiregional hypothesis. The oldest H sapiens skull in China was found in 1984. This, say Chinese archaeologists, indicates that the modern human and his ancestor, H erectus, may have existed side by side. Yet another possibility is that H erectus never moved out of Africa at all. Perhaps it was an even earlier ancestor, such as Australopithecus or H habilis, which moved out of Africa and then evolved into H erectus in Asia.

But this possibility is still in the realm of adventurous contemplation, and will remain so at least till another fossil find forces anthropological history to start all over again.