Nowadays we are the only species of human alive. However, this wasn't always the case. For most of hominin history, several human species co-existed. Just 40,000 years ago, for instance, we lived with Neanderthals 2.
This reached a peak ~2 million years ago when at least 7 different species of hominin lived together. Similar peaks in hominin diversity happened 2.5 and 3.5 million years ago 1.
These increases in diversity (and intervening drop off) are one of the big mysteries of our evolution. What happened to make our lineage so successful for a short while? And why did we fall off again afterwards 2?
Or maybe these boom and bust cycles never happened. That's the contention of new research, which claims this pattern is actually caused by us finding fewer fossils, not because there were fewer species 1.
Under their model, the human family was more consistently diverse; without the intervening busts 1. If true, it could mean that there are up to 20 new human species we haven't discovered filling in these gaps!
Missing human sites
This dramatic revelation is based on a seemingly simple observation published recently in PNAS (whose acronym sounds a little dirty if you sound it out) by a group of researchers working in London. Basically, they spotted that fossil sites are rarer during the dips in human diversity 1.
Could it be that we're just not finding key fossils during these periods? Or maybe, the fact that human fossil sites are rarer is just because there are actually fewer human species during those times.
So the researchers set out to see which of these ideas is correct. They looked at the total number of primate sites found in the past 5 million years, rather than just the number of sites with human fossils. This should give them a clearer picture of how thoroughly any given time period is studied 1.
Sure enough, they found that fewer fossil sites were studied during periods when fewer human fossils were found. This would suggest that the apparent drop in human species is actually just due to the fact we've studied that period less, not because there were fewer humans.
Climate change is fake news
Traditionally, the rise and fall in human diversity has been attributed to climate change. As new niches opened up, our lineage diversified to fill them. Then, as they disappeared many of those new specialists would go extinct. Only generalists, or the lucky, would survive.
This selection for generalists was, in turn, thought to be one of the reasons modern humans are so smart and adaptable. It's because that's what our ancestors needed to be to survive 2.
The discovery that the rise and fall of hominin diversity is fake news severely undermines this hypothesis. However, it could still have some truth. Maybe the environment still had a big part to play in our history.
So, armed with an improved understanding of our family's diversity, these researchers also looked to see if the climate had an impact on it. It turns out that it doesn't. The environment may well have influenced our evolution, but it doesn't seem to be the main reason species of human evolved or went extinct 1.
Missing human species
Armed with all these data, the researchers began to estimate how many human species there actually should be for any given period. These estimates show that there are many species out there we've yet to find that "plug in the gaps" of human diversity.
Their estimated pattern is very interesting, showing a gradual rise and fall in the number of human species over the past few million years. Peak humanity happened around 2 million years ago, when up to 8 species of human may have been alive at the same time.
What happened around 2 million years ago to cause this change in direction? Why did we start dying off? Perhaps finding some of the missing human species could tell us what's going on. Because these estimates imply there's a lot of unknown species out there.
In fact, it indicates there might be up to 20 human species we've yet to find. Talk about job security. It looks like I've got many years of blogging ahead of me.
References
- Maxwell, S.J., Hopley, P.J., Upchurch, P. and Soligo, C., 2018. Sporadic sampling, not climatic forcing, drives observed early hominin diversity. Proceedings of the National Academy of Sciences, 115(19), pp.4891-4896.
- Grove, M., 2012. Amplitudes of orbitally induced climatic cycles and patterns of hominin speciation. Journal of Archaeological Science, 39(10), pp.3085-3094.
