Filling the missing links

R. PRASAD
12 February 2009
The Hindu, (c) 2009 Kasturi & Sons Ltd

Intermediate forms are the stages through which animals evolved to become adapted animals.

In transitional forms one form may split to form another group without itself becoming extinct.

In 1859, when Charles Darwin published his “On the origin of species” there was great difficulty in interpreting the fossil record to substantiate his theory of evolutionary change. He was referring to the missing links in the fossil record to prove evolution.

Missing links are of two kinds - intermediate forms and transitional forms.
Intermediate forms mark the various stages through which groups of animals, say, birds or reptiles evolved to become fully adapted animals.

These forms should have the anatomical features intermediate between the two groups or species. Intermediate forms are well known in the case of humans. Humans evolved in various stages from their ancestor, the ape. But we do not see today the intermediate ones such as the Australopithecus afarensis and Homo erectus.

Transitional forms
The other missing link is the transitional forms. In the case of transitional forms what is being witnessed is that one form may split to form another group without itself becoming extinct. Take the instance of humans. Apes and humans split from a common ancestor about 7 million years ago. But we still find apes co-existing with humans. A transitional form, in the strict sense of the word, should just record the crucial aspects of evolutionary change as one group split from another.

People who oppose evolution point to the patchy fossil record showing the presence of transitional forms. One more ruse to shame evolution is to point out that apes, from whom humans evolved, are still seen today. Little do they understand the ways of evolution.

Archaeopteryx
The first transitional form — Archaeopteryx — was identified two years after he published his theory. Archaeopteryx is a transitional form between dinosaurs and birds.

Archaeopteryx had feathers, long, bony tail, and wishbone of birds but yet retained the reptile-like characteristics of pelvis and teeth. It had three claws on the wing which could have still been used to grasp prey.

Tiktaalik
Another classic example is Tiktaalik — a transitional form between marine (fish) and terrestrial animals. It was half fish, half amphibian.

It marked the first evidence of an animal that was on the verge of moving out of water to land. It was discovered just three years ago. Tiktaalik means a large shallow-water fish in the Inuit language Inuktikuk. Becoming a shallow water fish is one of the major steps in the transitional process.

Tiktaalik’s front appendages, for instance, end in a flat paddle, rather than fingers. Yet they were the first signs of fingers that we today see in tetrapods (the early limbed animals).

It was also the first fish to have a neck and an ear capable of hearing.

But it still had well-developed gills, fish-like scales, and a palate making it a half fish-half amphibian. Other transitional forms have also been identified prior to Tiktaalik.

Intermediate forms
Many important missing links have been identified in each group. Three years ago scientists found the remains of the nearly modern amphibious birds in the Gansu province of NW China.

Gansus, about the size of a pigeon but similar to diving ducks, is an important intermediate form between the oldest known bird and the modern ones.

Gansus, which was aquatic, indicated that modern birds may have evolved from animals that originated from aquatic environments.

The discovery last year of a frog, Gerobatrachus hottoni, settled the issue of origin of frogs.

The origin of today’s amphibians was till recently controversial as there were no intermediate forms. Add to it the fact that there was no transitional form either.
The Gerobatrachus fossil suggests that modern amphibians may have come from two groups - frogs, toads and salamanders have come from one and earthworm-like amphibians from the other.