New fossil trims our family tree

Our family originally evolved in Africa but over the last 2 million years we’ve spread throughout the world in a series of migrations. Dmanisi in Georgia is one of the oldest hominin sites outside of Africa; dating to around 1.78 million years ago. This makes it a key site for understanding what the first of these migrants – called Homo erectus – was like¹.

The skulls from Dmanisi. Skull 5 is the bottom one.

4 skeletons have already been discovered at Dmanisi and they’ve revealed that these early pioneers were surprisingly primitive; considering they were “advanced” enough to survive outside of their familiar African environment. They lacked the adaptable handaxes of later species and had very small brains, only slightly bigger than chimpanzees (600 – 700 cc, compared to the 450 cc of chimps)¹.  At the same time they were capable of adapting to the new plans and animals they encountered² and cared for their elderly. This raises all sorts of interesting questions over just how important big brains and advanced tools are.

Earlier this month information on a 5th skeleton (consisting of a complete skull with jaw bone and possibly a few bones from the rest of the body) from Dmanisi was published, providing even more information on the first non-Africans. However, this discovery also has far reaching implications for the rest of human evolution.

The new skull, imaginatively titled “skull 5”, was discovered in the same spot as the earlier finds so is most likely part of the same population. However, it is quite different from the first four skeletons, with a much smaller brain (546 cc) and a large projecting face. This shows that Homo erectus was a lot more variable than we first thought, changing how we should define the species³.

As the first migrants left Africa and spread throughout the world the populations in different regions began to evolve. Since these regional variants were first discovered in the 19th century there has been extensive debate over whether or not they were different enough to be defined as a new a species, or should all be labelled Homo erectus⁴. In fact when the Dmanisi skeletons were first discovered some wanted to call them Homo georgicus for this very reason.

The classic human family tree, with some of the species that may be removed by this find crossed off

By showing that Homo erectus was very variable, skull 5 definitively proves that these regional variants are not distinct species, but fall within Homo erectus. Further, Homo erectus isn’t the only species with “variants” that have been debated. The earlier Homo habilis (thought by some to the ancestor of Homo erectus) has a more robust version, thought by many to be a distinct species called Homo rudolfensis.

Since this new skull suggests hominin species are actually more variable, rudolfensis and habilis may have to be be re-classified as a single species. In fact, skull 5 shows that Homo habilis may even fall within the variation of Homo erectus³!  However, this conclusion is strongly contested by other scientists as the Dmanisi researchers only investigated a handful of Homo habilis skulls. But since we’ve only discovered a handful of skulls I’m not sure if this is a major problem; and I have a sneaking suspicion that even Homo habilis may eventually be grouped into Homo erectus.

All in all, this trimming of the human family tree may remove up to 3 different species; grouping them all into Homo erectus.

However, before we go rewriting textbooks some might be suspicious of whether or not skull 5 is even part of Homo erectus, given that it is so different to the others found at Dmanisi. Fortunatley, some bones from the rest of body were also found. These show that, skull aside, skull 5 was much more like the Homo erectus we’re used to with long legs and a more human-like body. It should also be remembered that the variation now documented within Homo erectus is no more extreme than the variation seen in modern chimps or humans³!

A comparison of the shape of the face (x axis) and brain size (y axis) of chimps, humans and fossil humans. Chimps are squares, humans circles and the numbers are the Dmanisi skulls. Note how the variation between chimps is no greater than the variation in the Dmanisi skeletons

So we should start trimming our family tree. This is a major change to our understanding of human evolution so has understandably been picked up by the popular press. Unfortunately they’ve run with headlines like “Skull of Homo erectus throws story of human evolution into disarray” (from the Guardian) or “Does this skull rewrite the history of mankind?” (from the Daily Mail). However, the “story” of human evolution remains unchanged and the skulls document the same evolutionary history. All this discovery does is change the labels a bit which – whilst significant – is hardly the “disarray” that some of the more extreme headlines suggest.

Skull 5 shows that single species of human are more variable than we thought. which means we have to trim our family tree; reclassifying some groups previously thought to be distinct species. At the very least, this will make it easier to learn the names of all of our ancestors!

References

1. Gabunia, L., Vekua, A., Lordkipanidze, D., Swisher, C. C., Ferring, R., Justus, A., … & Mouskhelishvili, A. (2000). Earliest Pleistocene hominid cranial remains from Dmanisi, Republic of Georgia: taxonomy, geological setting, and age. Science, 288(5468), 1019-1025.
2. Messager, E., Lordkipanidze, D., Kvavadze, E., Ferring, C. R., & Voinchet, P. (2010). Palaeoenvironmental reconstruction of Dmanisi site (Georgia) based on palaeobotanical data. Quaternary International, 223, 20-27.
3. Lordkipanidze, D., de León, M. S. P., Margvelashvili, A., Rak, Y., Rightmire, G. P., Vekua, A., & Zollikofer, C. P. (2013). A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science, 342(6156), 326-331.
4. Boyd, R., Silk, J. B., Walker, P. L., & Hagen, E. H. (2000). How humans evolved (Vol. 8). New York: WW Norton.