Emerging like family secrets, the fossils unearthed from the eroding wastes of East Africa are revealing that human evolution was a maze of promising beginnings and disappointing dead-ends.
The rootstock of human origins is more tangled than ever.
Since 1994, competing teams of scientists have discovered teeth, bones and skulls belonging to six previously unknown early pre-human species and subspecies. The fossils encompass the first 4 million years of human evolution. Three of the most ancient were made public this year.
Examined in the light of day, these time-stained remains are enough to confound the belief in a simple human family tree, experts said.
"Historically, everyone has always drawn a straight line: You had ancestors and then you had us," said Meave Leakey, the paleoanthropologist who recently announced her discovery of a pre-human genus and species dated to 3.5 million years ago.
"Now, people are more cautious. There are a lot of extinct branches. There is obviously a lot more diversity in the fossil record than we had ever reckoned," Leakey said.
Indeed, the recent findings reveal to researchers an unexpectedly diverse community of creatures on the cusp of becoming human, all apparently spurred by a single innovation: the ability to walk upright.
At almost every stage of prehistory, several species of homininds--as humanity's primitive predecessors are called--competed in an evolutionary struggle from which only one would ultimately emerge.
Since Charles Darwin first proposed his theory of human descent, researchers have identified more than 15 primitive species grouped in five distinct genuses, which all belong in the human family tree. They range from the relatively familiar, such as Neanderthal man, to the obscure, such as the recently discovered Homo antecessor. More species are almost certain to be discovered in years to come, several experts said.
No one agrees just how all these species are related to each other or which branch became the direct ancestor of the genus from which modern humanity sprang.
'Fossil Record Is Remarkably Complex'
"It does really suggest to all of us that the human fossil record is remarkably complex," said Donald C. Johanson at the Institute of Human Origins at Arizona State University. "I think it is going to get even more complex, with more branches discovered. There is probably diversity in every part of the human record.
"It is going to get very difficult to draw ancestor relationships."
The bones are evidence of evolutionary trial and error.
From the shape of a tooth, experts can discern the evolutionary leap from ape to man. In the curve of a single toe bone, they can trace the signature of upright walking.
Even so, no one knows for certain what first forced humanity's remote apelike ancestors to forsake the trees they called home, why some pre-human species surpassed others or why primitive human ancestors eventually migrated out of their African homeland to people the world. No one knows when pre-human animal calls developed into language, how hands molded to tools or how tool use was transformed into technology.
Not so many years ago, many experts in human origins were convinced that the earliest human ancestor was a single primate species best represented by a famous fossil skeleton known as Lucy, discovered by Johanson in 1974. It existed between 3 million and 4.2 million years ago. As best anyone could tell, that species roamed across Africa for almost 1 million years without significant physical change.
Then something abruptly spurred the developments that led to modern humans.
In more recent years, however, three groups of fossil hunters working independently in Kenya and Ethiopia have drastically rewritten that straightforward story of early human descent. Each of the six primitive hominid species they discovered offers the possibility of an entirely different opening chapter to human origins.
Reaching progressively deeper into the human past, the researchers uncovered:
* 2.5-million-year-old remains of a hominid designated Australopithecus garhi. The fossils were found in 1997 northeast of Addis Ababa in Ethiopia's Awash region by UC Berkeley paleontologist Tim D. White and his colleagues. These creatures may have been the earliest users of stone tools.
* 3.5-million-year-old remains of a new pre-human genus and species, designated Kenyanthropus platyops. Announced in March, the fossil was discovered near Lake Turkana in northern Kenya by Leakey and her colleagues at the Kenya National Museum. This creature, and not the species represented by Lucy, may have been humanity's ancestor.
* 3.9-million-year-old bones belonging to a primitive pre-human creature named Australopithecus anamensis. The fossils were found in northern Kenya by Leakey's team in 1994. The discovery of this species pushed the advent of bipedal walking back half a million years earlier than previously known.