Truth in Science

The platypus is an evolutionary enigma and we encourage you to read the new technical article that has just been posted on the Truth in Science Website. For according to Professor Tom Kemp:

The greatest mystery of all concerning mammalian evolution stretches back for 200 years: the question of what exactly the monotreme mammals are, and how they relate phylogenetically to therians [from T.S. Kemp (2007) The Origin and Evolution of Mammals Oxford University Press pages 173-174].

Mammals comprise three major groups. The therians [theria] include all placental mammals [eutheria] and marsupials [metatheria] which have the characteristic pouch in which the immature foetus is nurtured after birth. The third group of mammals are the monotremes or egg-laying mammals. These are the prototheria [as distinct from the theria] and there are only three species of monotremes living today, only found in Australia. These are the platypus [Ornithorhyncus], the short beaked echidna [Tachyglossus] and the long-beaked echidna [Zaglossus].

Is it a reptile?

At one time, it was believed that the monotremes were transitional between reptiles and mammals. It is easy to see why. The prototheria [Gk: first beast] lay small round eggs and have some other reptilian features. For example, the male platypus possesses a spur on its hind limbs through which it delivers a venomous cocktail which contains proteins that are similar to those found in snakes. Nevertheless, the main reason that monotremes cannot be considered intermediate between reptiles and mammals is the current Darwinian consensus that reptiles and mammals have evolved independently from a putative common ancestral amniote either via the synapsid [leading to mammals] or sauropsid [leading to reptiles and birds] lineages. We refer you also to the article “Synapsids and the Evolution of Mammals” which can be found here.

Since the recent publication [May 2008] of the platypus genome in the journal Nature there has been a great deal of activity in the academic press and in the popular media including the BBC.

Is it a bird?

All this activity, however, has not brought any resolution to the evolutionary enigma that is the platypus. If anything, the situation has become even more confused. Even though there is no doubt that the platypus is a mammal, there are many genetic features that are more like birds. For example, since the platypus lays eggs, there is an ongoing requirement for active yolk production and this is reflected in its genes. Perhaps the most remarkable feature of the platypus genome, however, is the structure and number of sex chromosomes. Typically, all male mammals have one X and one Y chromosome whereas females possess two X chromosomes. The male platypus has five X and five Y chromosomes and the female platypus five pairs of X chromosomes. In most mammals, the Y chromosome possesses a gene called SRY which is a major sex determining factor but this appears to be absent in the platypus. Sex determination in the platypus is therefore something of a mystery and a great deal of research is continuing in order to discover its mechanism. Elizabeth Finkel has written a recent review of this research entitled “Genome speaks to Transitional Nature of Monotremes” in the prestigious American journal Science in which she states:

The genome sequence now shows that one of the platypus X chromosomes [X5] has more than just that one bird gene: It's almost entirely equivalent to the chicken Z chromosome.

Of course, there are also other genetic features that are unique to the platypus. These include the possession of all the biology required for the exquisitely sensitive chemical and electrical detection systems in its leathery bill. In particular, researchers have discovered numerous genes coding for odour receptors. Similar genes are found in many other mammals that rely on a sense of smell, the dog being a classic example. The platypus, however, requires this sensitivity underwater.

Thus, the platypus remains an enigma. Is it from a sauropsid lineage which includes reptiles and birds? Is it from a synapsid lineage which supposedly led to the emergence of the mammals? Or is it derived independently from some unknown ancestral amniote? Or could it be that the Darwinian hypothesis, cladistic analysis or any other classification system for that matter is just far too restrictive? The platypus is a Darwinian cautionary tale. Is it a bird or is it a plain … old platypus?

To read the full article on the platypus – go here.