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Missing Evolutionary Link
The latest find — some well-preserved fossils of a fish that lived 375 million years ago — adds to the accumulating evidence that evolutionary forces acting over long periods of time can incrementally shape one kind of animal into another that looks quite different. It puts the lie to creationist beliefs that each species was created separately with its distinctive features already intact.
The newly discovered fish, described in scientific articles last week, was an ugly creature, four to nine feet long, with a flattish body, sharp teeth and a crocodile-like head, eyes perched on top. It was clearly a fish, with fins, scales, gills and a fishlike jaw. But it also had anatomical traits that anticipated the emergence of the earliest land animals: ribs strong enough to support a body no longer buoyed up by water, a neck that could move and limblike fins strong enough and flexible enough to allow it to haul itself out of the water.
Scientists speculate that this fish lived primarily in shallow waters and may have used its fins to push through vegetation-choked waters, lift its snout above water, clamber over logs or flop about clumsily on land much as seals do today. Why these or any ancient fish might have wanted to walk on land is not certain. They may have been looking for food or waddling to a nearby body of water or trying to escape larger predatory fish that could not follow them ashore.
Paleontology is sometimes denigrated as a backward-looking, historical science that cannot produce hypotheses that can be tested for truth or falsity. But the remarkable thing about this latest find is how predictable it was. Scientists knew what kind of creature they were looking for and where they would be apt to find it — in geologic sediments of the right age that were once at the mouth of a tropical river but had subsequently been moved by tectonic forces to Ellesmere Island in Canada, north of the Arctic Circle. They started looking in 1999, in a climate that was so forbidding that they could work only in the summer, guns at the ready to repel polar bears, and persisted until they finally hit pay dirt in 2004.
Scientists had previously found fossils of several other fish species that had attributes pointing toward land capabilities, as well as fossils of land animals that had residual features of fish, all suggesting the steps by which life forms moved from water to land. The latest fossils further round out the picture and provide compelling new evidence of how creatures that were mostly aquatic developed features that would later prove useful on land.
Some researchers suggest that the new species, named Tiktaalik (pronounced tik-TAH-lik) roseae, may become an icon of evolution, much like the fabled archaeopteryx, a protobird deemed to bridge the gap between reptiles (probably dinosaurs) and birds. That seems a bit of a stretch. Archaeopteryx had a powerful impact on our thinking about the evolution of birds whereas the new fossils mostly add to our previous understanding of how fish evolved into four-legged creatures known as tetrapods. Scientists will now be looking for fossils that are even closer to that transition point or that illuminate how tetrapods later evolved from a life mostly in the water to a life wholly on land. Stay tuned for discovery of the next missing link.