Species are what, exactly?
Most of us did some biology at school, and most of us came out with the idea that species are groups of populations that cannot interbreed. When we’re reminded of mules, which are the offspring of horses and donkeys, we think ‘Ah, but they are sterile, aren’t they?’ Almost invariably, although there have been a few cases of fertile mules, but when cattle and bison interbreed, while the male offspring are sterile, the female offspring are fertile. All the big cats can also mate with each other, producing hybrids (where the female is fertile), and in the case of the leopon, the hybrid between a leopard and a lion, even the male might be fertile.
Now that we can trace the ancestries not only of individual people, but whole populations and whole species, through DNA, it turns out that there has been a whole lot of successful – that is, fertile – interspecies breeding in the past. And it sometimes turns out that different species even today may interbreed with their neighbours on the quiet. For example, the primatologist Kate Detwiler discovered that two species of small monkeys in Tanzania’s Gombe National Park, the red-tailed monkey (Cercopithecus ascanius) and the blue monkey (Cercopithecus mitis), are found living in separate troops in some of the forested valleys, but in other valleys interbreed – in fact, in one or two valleys the monkey population consists entirely of hybrids.[i]
The idea that different species don’t interbreed is simply not true. They may not do so usually – but that is another thing entirely. We cannot use non-interbreeding as a criterion for species.
How then, can we define species?
For over 150 years now, the basic guiding principle of biology has been evolution – so the question we should be asking is what is the evolutionary status of species? The palaeontologist George Gaylord Simpson (1902-1984) suggested in 1961 that the essence of species is that they are evolutionary lineages. He got little reaction at the time because his colleagues largely were hung up on the non-interbreeding criterion, but from the late 1990s his insight has been more and more appreciated. The best way to recognise an evolutionary lineage is, quite simply, that it differs from other evolutionary lineages. Horses and donkeys differ consistently and therefore represent two separate evolutionary lineages, and are therefore two different species. Similarly, blue monkeys and red-tailed monkeys differ consistently and therefore constitute separate evolutionary lineages, and again represent two different species.
If there are whole populations which consist of hybrids between two species, then what? Sometimes hybrid populations remain isolated for a good length of time and become homogeneous – and a new species is born. At least one species of monkey, the stumptailed macaque (Macaca arctoides) of mainland Southeast Asia, is thought to have arisen about 1 million years ago from a hybrid population between two other species.[ii]
For a more detailed discussion about the arguments about how to define species, especially the contest between the Biological Species Concept and the Phylogenetic Species Concept, go here.
So … generally speaking, what are genera?
So what about genera, families and other taxa?
While the taxa at both ends of the ranking are pretty straight forward – ‘species’ is eminently useful, and ‘domain’ and ‘kingdom’ are irresistibly sensible – all the ranks in between can get awfully confusing. And they are actually rather arbitrary. When, for example, do we know that a group of organisms constitute a genus rather than a family?[iii]
One simple solution would be to organise those in-between ranks chronologically. In other words, the order Primates would include all those monkey, ape and human-like species which existed from the Palaeocene epoch, and the family Bovidae would include all those antelope, buffalo and cattle, and sheep and goat species which existed from the early Miocene epoch.
This is an idea first forcefully proposed by German biologist Willi Hennig (1913-1976), considered the founder of cladistics – or ‘phylogenetic systematics’ if your thesaurus is turned on.
In 1966, Hennig proposed linking the taxonomic rank of a clade to its time of origin. He argued that if taxa are to mean anything they must represent monophyla – that everything in that group must be descended from a common ancestor. He also argued that taxa had to be characterised chronologically.
Hennig was an entomologist and realised while many genera of insects separated from one another tens of millions of years ago, the genera of mammals and the genera of birds separated more recently.
The idea was taken up by American scientist Morris Goodman (1925-2010), one of the founders of molecular genetics. He set about constructing a consistent scheme for the group of mammals about which he was most familiar – the primates. In 1997, he suggested that a reasonable time depth for a primate genus would be seven million years, partly because this would do the least violence to the presently accepted system of determining genera.
This is where Colin Groves enters the story.[iv]
Colin surveyed many of the mammalian genera that taxonomists had recognised and found that most had separated from each other less than seven million years ago. Subsequently, he proposed that five million years was a more appropriate time depth for mammalian genera: the Miocene-Pliocene boundary.
Furthermore, Colin suggested that the taxonomic rank of ‘family’ had a time depth of 24 million years, separate families splitting around the time of the Oligocene-Miocene boundary. Going up one more ranking, the different ‘orders’ separated around the time of the Cretaceous-Tertiary boundary (the famous K-T boundary that marks the arrival of the asteroid that wiped out non-avian dinosaurs[v]).
One of the consequences of Goodman’s proposals for palaeoanthropology is that most if not all members of the human lineage would belong to a single genus. Indeed, using his original suggested time depth of seven million years, Goodman even included chimpanzees into Homo. Overall, the later modifications devised by Colin play less havoc with the established order, but they would still require that most human fossils be placed in the same genus as ourselves.
Arguments about when hominins evolved into a genus that can be described as wholly human traditionally revolved around the relative importance of different physical characteristics: brain size, dentition, general morphology (body size, especially the extent of sexual dimorphism), and primary form of locomotion.
Other more controversial factors sometimes taken into consideration include tool-making, art and other signs of culture, and evidence of community living.
For example, some experts such as Ian Tattersall, curator emeritus with New York’s American Museum of Natural History, argue that the cranium of Homo floresiensis (the Hobbit, see here, here and here) is too archaic for it to be included in our genus.
This leads us to our second, and more controversial opinion: following Colin’s plan our genus would include not only H. floresiensis but even older and more archaically featured species traditionally belonging to other genera, such as the Australopithecines, for example, which include the Taung Child and Mrs Ples.
Colin argued that the Miocene-Pliocene boundary more or less corresponds to the onset of the only characteristic definitely belonging solely to our genus and to no other genera among the great apes – bipedalism. By bipedalism we mean that the main form of locomotion is walking or running on two legs, with the big toe aligned with the other toes in the foot.[vi]
Accepting this argument has two major implications and several minor ones for palaeoanthropology. First, and least controversially, brain size is not by itself a qualification for membership of the human genus. Specifically, a small brain does not exclude membership.
The discovery of H. floresiensis and H. naledi in the 21st century, with an average brain size of around 420 cm3 (about the size of a modern chimpanzee) and 500 cm3 respectively, clearly demonstrates that many humans were small brained compared to H. sapiens but possibly still capable of sophisticated tool-making and ritual behaviour.
Secondly, accepting a time criterion in determining what species do and do not belong to the genus Homo means that strictly morphological traits are no longer intrinsic in determining human status.
In the next post, we’ll look in more detail at brain size, culture and bipedalism as criteria for determining whether or not a species is human.
Other posts in this series can be found here:
‘Us’ Part 4 – Using your noggin
[iii] Sigwart, J., Sutton, M. D., & Bennett, K. (2017). ‘How big is a genus? Towards a nomothetic systematics’. Zoological Journal of the Linnean Society. https://doi.org/10.1093/zoolinnean/zlx059
[iv] Groves, Colin. ‘Time and taxonomy’. Ludus Vitalis. Vol IX. No 15. 2001.
& Groves, Colin. ‘Speciation in hominin evolution’. African Genesis: Perspectives on Hominin Evolution. Ed Reynolds, Sally & Gallagher, Andrew. Cambridge University Press. 2012.
& Groves, Colin. ‘Current taxonomy and diversity of crown ruminants above the species level’. Zitteliana B32, International Conference on Ruminant Phylogenetics, ed. Prof. Dr G Worheide, Bavarian State Collection for Paleontology and Geology, Munich.
[v] Now also sometimes referred to as the Cretaceous-Palaeogene (K-Pg) boundary.