IT'S BEEN A TOUGH month for creationists. On April 6, evolutionary biologists announced the discovery of a fossil of \o7Tiktaalik roseae\f7, a giant fish whose fins were evolving into limbs when it died 375 million years ago. This scaly creature of the sea was in transition to becoming a land animal, the discoverers wrote in Nature.
A day later, molecular biologists reported in Science that they had traced the origin of a key stress hormone, found in humans and all vertebrates, back 450 million years to a primitive gene that arose before animals emerged from oceans onto land.
Both teams of scientists stressed that their findings contradicted creationists -- and demonstrated how small, incremental steps over millions of years could indeed produce complex life, ranging from the intricate mechanisms of a hormone molecule to the assembly of limbs from fins.
But even as they were touting their results as yet another validation of Charles Darwin's theory of evolution, biochemist Michael Behe, a leading advocate of "intelligent design," dismissed the hormone discovery as "piddling."
As if in response to Behe's challenge, paleoanthropologists raised the stakes last week with yet another example of evolution unfolding in our own lineage. In the journal Nature, a team of researchers from UC Berkeley and Ethiopia found an "intermediate" member of the human family that they say unambiguously fills the gap in the fossil record between two early types of human ancestors. \o7Australopithecus anamensis \f7was a creature the size of an orangutan that walked upright in the Rift Valley of eastern Africa about 4 million years ago, more than 2 million years after the human lineage split from the ancestor we share with chimpanzees
The team found the species in a mile-deep stack of sediment in northeastern Ethiopia, which has become the Comstock Lode of human evolution. Across 11 separate layers, researchers unearthed several types of early human ancestors, with the \o7anamensis\f7 bones sandwiched between layers containing two other species -- \o7Australopithecus afarensis\f7, the species whose most famous member was the diminutive skeleton Lucy that lived 3.2 million years ago; and the 4.4-million-year-old \o7Ardipithecus ramidus\f7. (They also found the oldest known member of our species, Homo sapiens, in a layer dating back 155,000 years.) When researchers compared the teeth and bones of these various human ancestors, they saw a clear path from primitive to modern.
