Saturday, April 08, 2006

WSJ Science Journal: Two New Discoveries Answer Big Questions in Evolution Theory

Two New Discoveries Answer Big Questions in Evolution Theory:

Even as the evolution wars rage, on school boards and in courtrooms, biologists continue to accumulate empirical data supporting Darwinian theory. Two extraordinary discoveries announced this week should go a long way to providing even more of the evidence that critics of evolution say is lacking.

One study produced what biblical literalists have been demanding ever since Darwin -- the iconic "missing links." If species evolve, they ask, with one segueing into another, where are the transition fossils, those man-ape or reptile-mammal creatures that evolution posits?

In yesterday's issue of Nature, paleontologists unveiled an answer: well-preserved fossils of a previously unknown fish that was on its way to evolving into a four-limbed land-dweller. It had a jaw, fins and scales like a fish, but a skull, neck, ribs and pectoral fin like the earliest limbed animals, called tetrapods.

Discovered in 2004 on Canada's Ellesmere Island by Neil Shubin of the University of Chicago and Ted Daeschler of the Academy of Natural Sciences in Philadelphia, the 375-million-year-old Tiktaalik roseae "blurs the boundary between fish and land animals," said Prof. Shubin. It "is both fish and tetrapod," showing how life made the transition to land, evolving four limbs from fins.

Previously known fossils of ancient "lobe-finned fish" also seem poised between fish and tetrapods, with pectoral fins containing precursors of the humerus, radius and ulna of tetrapod armbones. But Tiktaalik (an Inuit word for shallow-water fish) makes a stronger case. Its pectoral fin still has thin, fish-like bones, but also contains the three armbones-to-be as well as a wrist-like structure and a hand-like one. The shoulder and elbow could bend, and the proto-wrist could extend, allowing the fin to support the body and propel it on land. "Tiktaalik shows us the stages in the evolution of the tetrapod body plan," says Dr. Daeschler...

...Another discovery addresses something Darwin himself recognized could doom his theory: the existence of a complex organ that couldn't have "formed by numerous, successive, slight modifications," he wrote in 1859.

The intelligent-design movement, which challenges teaching evolution, makes this the centerpiece of its attack. It insists that components of complex structures, such as the eye, are useless on their own and so couldn't have evolved independently, an idea called irreducible complexity.

Because only functional structures survive, they say, useless components such as parts of an eye couldn't lie around for eons waiting for dumb luck to assemble them into a (finally) functional unit. These complex structures therefore must have been assembled by a designer.

One such complex structure is a hormone and its receptor. Just as a keyhole has no use without a key and vice versa, a hormone is useless without a receptor that lets it dock with a cell, and a receptor serves no purpose without hormones. Catch-22: Neither component could survive without the other, yet it strains credulity to suppose that both structures popped onto the evolutionary scene simultaneously.

To investigate this puzzle, biologists led by Joseph Thornton of the University of Oregon reconstructed an ancestral receptor. They first analyzed receptors for steroid hormones in 59 species, including primitive jawless fish and skates. Then, in a process called gene resurrection, they worked backward to infer what the gene for the ancestral receptor was, and actually made the receptor in the lab: a molecule that last existed on earth 450 million years ago.

Update: Breakthrough of the Year: Evolution in Action:

Equipped with genome data and field observations of organisms from microbes to mammals, biologists made huge strides toward understanding the mechanisms by which living creatures evolve...


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