Tuesday, July 22, 2008
In what is one more body blow to those who question Darwin's theory of evolution, scientists recently have discovered well-preserved fossils of a fish that provide the vital transitional link between fish and land dwelling four-legged vertebrates — tetrapods. One of the many objections to Darwin's theory is based on the apparent lack of transitional links and intermediate forms in fossil records to prove conclusively that evolution had ever occurred. But the discovery of three well-preserved nearly complete skeletons of Tiktaalik roseae, "an intermediate between fish with fins and tetrapods with limbs" has proved the sceptics wrong. Tiktaalik, with its many morphological and functional changes, has provided the much sought after scientific evidence to the transition of fish to land living animals. Found in sedimentary rocks belonging to the Late Devonian period, 375 million years ago, in the Canadian Arctic region, the fossils show characteristics of a shallow water river deposition in subtropical to tropical climatic conditions before drifting continental plates carried them to their present habitat. Tiktaalik fits well between the earlier evidences of fish able to walk in shallow water that lived some 385 million years ago and fish with limbs bearing digits that lived about 365 million years ago. Shallow water has been found to play a pivotal role in the evolutionary changes; the fish developed a new mechanism of head movement, respiration and body support system, thus allowing Tiktaalik to exploit both water and sub-aerial habitats.
Tiktaalik provides a vital missing link in the fish to tetrapod transition spanning a period of about ten million years. For a transitional fossil to fit snugly between existing fossil records, it should resemble as well as differ from those immediately higher and lower down in the phylogenic tree. Tiktaalik has many morphological features that are intermediate between its nearest neighbours. In fact, it resembles a poorly preserved animal that belongs to a lower order in the evolutionary tree. The discovery of Tiktaalik can be likened to the finding of proto-bird Archaeopteryx, considered a breakthrough in tracing the transition from reptiles to birds. Despite the finding of a vital link, there still exist lacunae in fossil records tracing the transitional changes going up to tetrapods. Tiktaalik found in the Canadian Arctic, one of the most inhospitable terrains, is proof that rocks do contain within them many secrets waiting to be uncovered. The answer lies in searching for the right rocks in the most probable depositional environment and geological time period for the missing link, just the way scientists zeroed in on the most probable rocks while looking for Tiktaalik fossils. The latest discovery of Australopithecus anamensis, a transitional link in the evolution of humans in the Middle Awash region in northeastern Ethiopia, which is also home to seven other human-like species, is yet another example.