Looking around at the natural world, have you ever wondered why some groups of organisms contain huge numbers of species while others are seemingly barren?
Take insects as an example, animals which evolved around 480 million years ago. There are perhaps 6 million species living in all manner of environments, and occupying an incredible diversity of niches. Surprisingly though, they have never truly adapted to the marine environment.
Contrast this with Methanopyri, in the Kingdom Archaea, for which there is only a single species (Methanopyrus kandleri) which evolved close to 4 billion years ago.
This remarkable bacterium was found living on the edge of a ‘smoker’ under extreme conditions: 81-110 degrees celcius, high carbon dioxide concentration and at a depth of 2,000 metres in the Gulf of California.
Just why some groups contain large numbers of species while others don’t has long puzzled biologists. One of the main explanations has been geological age - older groups of organisms are more diverse because they have simply had more time to accumulate greater numbers of species.
Yet, the fact remains that some comparatively young groups of species are remarkably diverse; and conversely, some like the Methanopyri are very ancient but species poor.
A new study by Joshua Scholl and John Wiens published in the Proceedings of the Royal Society B has taken a fresh look at this age old problem.
Why is there so much variation in bugs, and not in humans? irin-k/Shutterstock
They looked for the first time ever at the rates at which new species were formed across the entire Tree of Life, rather than just a subset of organisms as has been the focus until now.
They found some remarkable and fascinating patterns that shed new light on the question of diversity and its possible causes.
Over the course of life’s history, plants have had a species production rate more than twice that of animals, while complex organisms (multicellular eukaryotes) have produced new species at a rate almost 10 times that of simpler one (protists and prokaryotes).
The work could also help explain another long held mystery: why did sexual reproduction evolve? Sex seems to have been a major catalyst for increasing the rate at which new species formed, perhaps explaining its success as an evolutionary strategy.
Among the vertebrates, a terrestrial lifestyle seems to explain greater species diversity. While simply living in a marine versus non-marine habitat might be the major reason for high species number in some major invertebrate groups, like molluscs.
Back to insects, adopting herbivory was probably the key to explaining high rates of new species forming in the past and their remarkably diversity today.
All of this made me pause and reflect on our own group of species, the two-footed apes, or hominins, and our incongruous existence today.
It’s striking that we find ourselves alone, especially when we contrast this with the remarkable diversity of hominins seen in the past. Might this tell us something about humans today, and perhaps even where we might be headed as a species?
Our broader biological group, the Order Primates, contains the lemurs, lorises, tarsiers, monkeys and apes. There are around 350 species of living primates in a group that evolved perhaps 80 million years ago. Today, we’re quite a diverse lot, with primates representing somewhere around 5 per cent of the total number of mammal species.