A short article appeared in NewScientist online on archaeans, a microbe with a prokaryotic cell similar to bacteria. Apparently in the varieties of archaeans on which this particular research was carried out, it was not possible to categorize them into separate species, due to extensive lateral gene transfer i.e. varieties of archaeans were swapping genes between them to an extent that they were not genetically distinct enough to be called separate species.
What are species? The term is so commonly used that it comes as a surprise to learn that this has been a vexing question for biologists to answer. Species are interbreeding aggregates of populations that are reproductively isolated from other such groups due to physiological or behavioral barriers. In asexual organisms this definition runs into an obvious problem which is that these organisms do not interbreed, they are clones. Species in these cases are recognized on the basis of ecological preference and or morphological differences. Extinct organisms pose another problem. Here, the recognition of fossil species is on the basis of morphological differences, the rationale being that if populations show enough morphological differences then individuals belonging to these different populations would not or could not have interbred. But how much difference is enough? The literature on species concepts is large. I found Keywords in Evolutionary Biology to be one of the best reference books. Recently, this blog raises interesting questions on the concept of species.
Okay, on to Archaea. The article in NewScientist begins with the sentence:
"Some primitive organisms may be impossible to divide into species".
Are Archaea primitive? Primitive as in early inhabitants of earth, or primitive as in "less evolved"? To paraphrase Bill Clinton, it depends on what the meaning of the word "primitive" is is. Below is the phylogeny of life (source: Wikipedia)
Bacteria, Archaea and Eucaryota (the group we belong to) form three major domains of life. These three domains have had more or less the same time to evolve since their descent from the universal common ancestor. Some biologists have argued in favor of a different phylogeny, one in which Archaea and Eucaryotes evolve from a variety of bacteria, but you get the point. To say one living group of organisms is more primitive as in "less evolved" than any other is meaningless. It is not as if Archaea originated early in the history of life and stopped evolving. All organisms have primitive and derived traits, i.e. some properties which were inherited from their ancestors and conserved or were not changed much, and derived properties that arose later in the history of that lineage. So, for example certain aspects of cell physiology in humans is regulated by molecular mechanisms that are nearly identical with other vertebrates. This is a primitive trait, that must have arisen in the common ancestor of vertebrates and has not changed much since. Other properties such as our bipedal gait evolved uniquely in our lineage later. This is a derived trait. Archaeans must also contain such primitive traits that they inherited from ancient archaeans and some derived traits which must have evolved more recently.
Biologists have long since discarded the concept of scala naturae, the linear arrangement of life forms from the lowest presumably bacteria, to the highest- us humans. But in popular imagination, we remain bound to this great chain of being.