The term 'missing link' is often used in the popular press, and equally often deplored by biologists as a rather glamorous over simplification. It refers to an organism with features intermediate between two existing classes of living organisms, and is thought of as a bridge between them during the course of evolution. In actual fact, the existence of organisms with features of two different classes of animals is both exciting and important to our understanding of organic evolution, but rarely has it been reduced to the level of a single animal. Rather, several animals with features transitional between two currently existing classes of animals have been discovered from the study of fossils found all over the world. Sometimes, these represented paths that explored a novel form which did not evolve further into anything else, what is referred to as an 'evolutionary dead end'. In other cases, some features have evolved many times over evolutionary time (for instance the ability to fly, instinctively linked with birds by a layperson, has evolved many times, among insects, birds and mammals as well) and so cannot be used as sole evidence to speak of 'the' missing link.
The theory of evolution by natural selection put forth by Charles Darwin and Alfred Russel Wallace speaks of a gradual transition from one living form to another over the four billion years or so that life has existed and diversified on earth. This would mean that there must be animals that look partly like amphibians and partly like reptiles, and so on for all closely related classes of living beings. However, from our own experience, the living world is composed of distinct species and types of animals and plants. On encountering an unfamiliar creature, we are unlikely to be confused about whether it is a bird or a monkey. Where then are these intermediate creatures that show the transitional forms between existing and clearly defined species? The place to look for organisms that once existed but are no longer represented in the current world, is among fossils. The fossil evidence shows that several intermediate species were not as successful in establishing themselves as the two species 'flanking' them and went extinct. Darwin himself was the first to lament the incomplete nature of the fossil record. A chapter of his famous book 'On the Origin of Species' is in fact entitled 'Absence or rarity of transitional varieties'. In this he says "If my theory be true, numberless intermediate varieties, linking closely together all the species of the same group, must assuredly have existed; but the very process of natural selection constantly tends, as has been so often remarked, to exterminate the parent-forms and the intermediate links. Consequently evidence of their former existence could be found only amongst fossil remains which are preserved".
However, although the fossil record is by no means complete, over the years palaeontologists have discovered several interesting and amazing creatures as fossils. Perhaps among the most popular examples is the flying dinosaur, the archaeopteryx, with the feathered wings of a bird but carrying a tail and teeth that were like those of dinosaurs. It was hailed as the oldest bird and the possible link between reptiles and birds. This with particular fervour, as it was discovered in 1861, just two years after Darwin published “The Origin of Species”. After this however other bird-like dinosaurs and dinosaur-like bird fossils have been unearthed, blurring the special status of archaeopteryx.
Another fascinating organism that has been hailed as a missing link is the coelacanth. Interest in the coelacanth has recently resurfaced as its genome sequence was worked out and made public just two months ago. The coelacanth is special among all those fossils hailed as 'missing links' because it suddenly emerged as a live specimen off the coast of East Africa in 1938. Samantha Weinberg in her book "A fish caught in time" describes the events leading up to and following its discovery. The story reads like a thriller. In 1839, the Swiss scientist Louis Agassiz described the fossil of an unusual fish tail found in the north of England. He named it Coelacanthus granulatus, because the fin rays were hollow (Coelacanthus is Greek for hollow spine) and the scales appeared to be decorated with tubercles (granulatus). After this, several Coelacanth-like fossils were discovered all over the world, from China to Brazil to Madagascar. All of them had the same features but were vastly different in size, ranging from a few centimetres to as much as three metres. The oldest fossil dated back to about 400 million years (the Devonian period), while the most recent was about 70 million years old (the Cretaceous period). It was generally assumed that the creatures had become extinct around 70 million years ago along with the dinosaurs. It was known that sometime in the end of the Devonian period, a species of freshwater fish evolved limb like structures and moved on to ‘conquer’ land. There were three main candidates for such 'Ichthyostega' or 'walking fish': the lungfish, the rhipidistian and the coelacanth. But poring over fossils it was hard to determine which was the best bet, as the search was both for limb-like structures as well as the ability to breathe on land, that is the presence of lung like structures. It appeared that a study of the soft parts was necessary. In December of 1938, Marjorie Courtenay Latimer, working as a curator in the East London Museum in South Africa, was brought a pile of fish to sort for the museum. This was part of the routine specimen collection for mounting and display. She discovered a blue green fish of five feet with an iridescent sheen and a 'funny puppy dog tail'. Although she didn’t know what it was, its appearance was sufficiently unusual to contact an expert. This first live specimen of a coelacanth survived many near disasters: Marjorie Latimer's near rejection of the fish pile, the lack of a cold storage facility, the scarcity of formalin to preserve it and the delay in response from the expert, J.L.B Smith. As a result the soft parts could not be saved, and there was another hunt for a living coelacanth. Finally one was found off the Comoro islands between Mozambique and Madagascar after nearly 15 years. This species of coelacanth was named 'Latimeria chalumnae' in honour of the discoverer.
Was this then a great-great..great grandparent of the first land vertebrate? Studies comparing lungfish and coelacanths tended to support the lungfish as closer than the coelacanths to the first vertebrate on land. However, because similarities between genes are believed to be clues to relatedness between groups of animals (phylogeny), the Coelacanth genome sequence was eagerly awaited. Lungfish genomes are very large, and unwieldy to deal with. They contain about 100 billion base pairs of DNA. The Coelacanth genome is about the same size as the human genome, about 3 billion base pairs. The complete sequence of the African coelacanth (today we have another species, the Indonesian Coelacanth) was worked out two months ago and the study published in the journal Nature. The study compared sequences of genes involved in limb development (as this was an important element of living on land) as well as regulation of genes involved in building a body plan compatible with life on land. It also compared several gene sets across lungfish from the limited amount of sequence data that is available, tetrapods (representative four legged vertebrates) and coelacanths. The result of this comparison showed that indeed the lungfish are more closely related to early land animals than are coelacanths. In order to assess the rate at which the coelacanth genome has evolved, the authors calculated a 'substitution rate' which estimates how frequently the coelacanth genome has undergone mutations over evolutionary time. The conclusion was that the coelacanth genome has been evolving at a significantly slower rate than the genes of lungfish and tetrapods. In fact the authors suggest that the strange prehistoric appearance of the current day coelacanth is because of the slow rate of evolution of its genome. This is a somewhat surprising result, because in other aspects the coelacanth genome appears to be similar to its lungfish or tetrapod counterparts. Critics argue that because all genomes contain regions that change more frequently than others, the choice of gene sets that have been compared could have biased the findings . Without knowledge of how the coelacanth genome is regulated, it is not easy to determine which genes should be compared with which between lungfish and tetrapods. However, it does appear clear that coelacanths are not the immediate ancestors of the first land vertebrates. Lungfish are the closest we have to such a link today; future fossils may lead to an even closer link. Be that as it may, by existing today coelacanths have enriched our knowledge of our own past and the world around us by their dramatic appearance and prehistoric looks. As yet there remain precariously few - about 400 individuals- in their African hideout. It is up to us to ensure that they remain safe.
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