New Insights into Human Origins - Part 3

Kia ora,

It's hard to believe that another year has drawn to a close. With regards to the contemporary world, 2021 was certainly a year that presented many challenges as the global COVID-19 pandemic continued. However, I don't want to continue to dwell on all of that in today's post - but instead provide a brief and hopefully interesting respite by looking back to hundreds of thousands of years ago.

As I have previously mentioned on this blog a keen area of interest of mine is the study of human origins and despite global restrictions on travel and fieldwork last year, there was a whole heap of relevant research reported in 2021.

What would I pick as my favourites? It really is hard for me to go past what I consider to be an outstanding technological and methodological achievement - a " 'game changer' in the field" as I described it in a post back in April 2021 - the successful extraction and analysis of nuclear DNA from a number of species, including hominins, from Pleistocene cave sediments (i.e. dirt!). This achievement was recognised by Science magazine as one of the top breakthroughs across all fields of science in 2021.  

Another personal favourite would have to be the announcement of a fossil skull discovered in the city of Harbin in the Heilongjiang province in northeast China. In a separate paper published in the same issue of the scientific journal 'The Innovation' some of the Chinese researchers behind the discovery suggested that the Harbin skull should be recognised as a new species - Homo longi (according to the paper, the species name longi is derived from the geographic name Long Jiang, a common usage for the Heilongjiang province and which translates to English as "dragon river"). Sure, the naming of a new species of hominin is always enough to draw attention (and invariably some debate!), but there are other reasons why this particular discovery appeals to me so much. For starters, the fossil is such a beautifully preserved skull. At the risk of upsetting those who specialise in the extremely important study of post-cranial elements of hominin skeletons there is (in my humble opinion) nothing quite as alluring as a well preserved skull. I mean, just look at it!:

Anterior (A) and lateral (B) views of the Harbin skull. Image sourced from Qiang Ji et al. 2021.  

The story behind the discovery of the Harbin skull is also fascinating. It was reportedly first discovered in 1933 during the construction of a bridge over the Songhua River in Harbin. At the time the city was under the occupation of the Japanese and the Chinese worker who discovered it hid it down a well to keep it out of the hands of the Japanese occupiers, only revealing its existence to family just before his death over 80 years later. His grandchildren retrieved the skull and subsequently donated it to researchers at Hebei GEO University in 2018. Unfortunately, as a result of this history, the precise location and therefore stratigraphic context of the Harbin skull when it was first discovered was uncertain, impeding efforts to precisely date it. Some geochemical analyses were undertaken to identify the most likely layer of origin - a bed of lake sediments deposited between 309,000 and 138,000 years ago. In an effort to try and narrow that window of time further the skull itself was also dated using a method called Uranium Series (or U-series) dating. Like the better known radiocarbon dating, U-series dating makes use of the known rate of the natural decay of radioactive isotopes. Specifically, the change of Uranium 234 into Thorium 230 as the result of the loss of a particular particle during what is actually a much longer decay sequence, or series (you can check out the U-series hyperlink above if you are interested in the whole thing). A drawback of this process was that it required the destruction of tiny samples of the skull. The result was perfectly consistent with the age estimate window above, strengthening the case for the match to sediment layer, but unfortunately didn't help narrow that window by much. The U-series dating provided a MINIMUM age of 146,000 years for the Harbin skull, but its true age could actually be considerably older - potentially as old as 309,000 years old based on the dates of sediment deposition mentioned above. 

Finally, an intriguing and especially exciting possibility that has been entertained by a number of researchers is that the Harbin skull may finally allow us to put a face to the enigmatic group of hominins presently known as Denisovans! To recap from a previous post, the Denisovans are a sister group to Neanderthals (Homo neanderthalensis) that are currently only known from DNA and a few very fragmentary fossils recovered from sites in Siberia and on the Tibetan Plateau and dated to between about 200,000 and 30,000 years ago. This possibility, however intriguing, will remain little more than speculation until DNA or, alternatively, proteins can be successfully extracted from the Harbin skull. 

Could this be the face of a Denisovan? Life reconstruction of the Harbin skull. Image sourced from Xijun Ni et al. 2021.

Thanks for reading,

Nick