(This article was published on November 6, 2016 12.40pm SAST in The Conversation)
How many species of humans have existed? It all depends on the concept of species that’s being employed. In some approaches, there was – and still is – only one. In others, there are as many as 17 species of Homo.
This is because taxonomy, the process involved with the discovery and classifying of species, has been contentious since time immemorial.
Even pre-scientific “ethno-taxonomists” had what ethnobiologist Brent Berlin has called a “largely unconscious appreciation of the natural [biological] affinities among groupings of plants and animals … quite independently of [their] actual or potential usefulness or symbolic significance in human society”.
Our Afrocentric experience with plants and animals supports this. The Akamba people from Kenya partition species relatively broadly. Nzoka are snakes; nyunyi are birds and nyeki are grass-like plants. In Malawi the Tumbuka, Chewa and Lhonwe independently recognise the same “species”. They partition these more finely than the Akamba do, down to the equivalent of biological genera. In South Africa, the same is true of northern Sesotho speakers for both trees and birds.
The bottom line is that all humans have an innate interest and ability in naming biologically meaningful entities. Taxonomy, then, vies for the title of world’s “oldest profession”.
Typological and Darwinian species
Before Charles Darwin, nearly all scientists believed that life on earth, including humans, was created by God thousands of years earlier and had remained unchanged over time.
Working on this premise during the 1700s the “father of taxonomy”, Carolus Linnaeus, used morphology –overall internal/external physical form – to describe species. Linnaeus named us Homo sapiens in 1758. Today, the Linnaean system of classification remains the basis for naming all life forms.
Then came 1859 and Darwin’s On the Origin of Species. In it, Darwin demolished the notions that life on earth was created only a few thousand years earlier and that species are immutable. However, he took no clear position on what constitutes a species:
There is no infallible criterion by which to distinguish species and well-marked varieties … The opinion of naturalists having sound judgement and wide experiences seems the only guide to follow.
In short, Darwinian species are artificial constructs partitioning an evolutionary continuum. We, like most neo-Darwinian evolutionary biologists of the 20th and 21st centuries, disagree.
But Darwin’s thinking, based on the premise of “descent with modification”, laid the foundations for an evolutionary concept of species that allows their placement in phylogenies or evolutionary genealogical trees.
Limitations and alternatives
Pre- and Darwinian species concepts have limitations. Some pre-Darwinian taxonomists – and even the man himself – maintained that interbreeding between anatomically distinct, “good” species only warrants their rank as subspecies or races.
“Proto-ecologists”, meanwhile, suggested that anatomically similar populations which differ in ecology and behaviour form valid “cryptic” species.
To deal with these concerns, taxonomists in the early 20th century adopted the reproduction-based Biological Species Concept. These are real, self-defining, protected gene pools, irrespective of anatomical distinctiveness. They are separated by intrinsic pre-mating – such as male and female displays – and/or post-mating reproductive “isolating mechanisms”. These include embryonic death and offspring sterility.
Not all modern taxonomists were satisfied with this concept. Since 1950, 26 rival concepts of species have emerged.
Based on our own lived experience in Africa, we prefer a novel approach called the Consilience Species Concept (CSC) to deal with the issue of what constitutes a species.
A different approach
The Consilience Species Concept incorporates useful features from other concepts. A CSC-species is a group of populations that can be identified using a suite of heritable, complementary, arguably independent characteristics. These include qualitative anatomical, behavioural, ecological, physiological and molecular genetic features. All of these features show consilient, multifaceted variation.
The term consilience – a “jumping together” of knowledge – was coined by philosopher William Whewell. To put it simply: if it walks like a duck, quacks like a duck; water rolls off its back, it has webbed feet and a flattened bill, it’s a duck.
In the CSC approach, then, academic, conservation and citizen scientists should consider evidence from a range of independent sources. They should then delineate species on the basis of consilient relationship and not on ability to interbreed (or not) or some arbitrary amount of difference in anatomy or DNA composition.
The CSC is superior to its competitors because, by design, it prevents the recognition of huge numbers of trivial entities. It doesn’t ignore evolutionarily significant ones because they interbreed. It can also be applied consistently to both sexual and asexually reproducing “species”.
What is particularly compelling about the CSC concept is that it constitutes novel, Afrocentric science. It both challenges and assimilates Eurocentric ideas.
The Eurocentric Darwinian and reproductive-isolation-based Biological Species Concepts for species and subspecies just don’t deal adequately with species as whole, functionally interconnected entities. The CSC has its philosophical roots in the African sourced “holism”, which was elucidated by the South African statesman and philosopher Jan Smuts in the 1920s. In short, it argues, “The whole is more than the sum of its parts”. This is, as we’ve shown, very true in the complex world of taxonomy.
Potiphar M. Kaliba, the director of Museums of Malawi and a PhD graduate from the University of Cape Town, was also among the co-authors of this article.