Our Evolutionary Past: Branches Through Time

Kia ora,

This past weekend I took advantage of a quiet Saturday and headed along to 'Our Evolutionary Past: Branches through Time', a new temporary exhibit that had just opened at the local Otago Museum. 

(Note: In this blog post I have included a number of hyperlinks that you can follow for more information, including links to original research papers. Unfortunately, however, many of the research papers I refer to are behind paywalls!)

This free exhibit, scheduled to run until June 10 2021, focuses on some key themes in human evolution, such as the emergence of uniquely human traits like obligate bipedalism (walking on two legs as our primary method of getting around) and manual dexterity (the ability to use our hands to preform extremely fine movements such as writing or threading a needle by precisely grasping and manipulating objects). Highly accessible educational content combined with some great artwork by Dunedin artist Bruce Mahalski is showcased on massive floor-to-ceiling digital projections. There's an activity area to keep younger children occupied as well!

Fossil representatives of some of the earliest known hominin species. From left to right: 'Ardi' (Ardipithecus ramidus); 'Lucy' (Australopithecus afarensis); 'Mrs Ples' (Australopithecus africanus). Artist: Bruce Mahalski. 

From left to right: Homo neanderthalensis skeleton; the 'Kabwe Skull' (usually attributed to the species Homo heidelbergensis); 'Denisovan' mandible from Baishiya Karst Cave on the Tibetan Plateau (see my previous post for more about Baishiya Karst Cave).
Artist: Bruce Mahalski.

In a display case at the front of exhibit sits the only fossil cast in the gallery - the infamous scientific hoax known as 'Piltdown Man' - at first glance an unusual choice for inclusion in such an exhibit! 

  

Cast of the Piltdown Man skull.

But as noted in the accompanying information panel, the story of 'Piltdown Man' serves as a reminder of how far our understanding of human evolution has advanced since the skull's 'discovery' in England in 1912. At the time there was a ready acceptance of the ape-like jaw and human like braincase as the long sought 'missing-link' between apes and humans by many in the British scientific establishment because it fit perfectly with the widely held belief at that time that a large brain was one of the first human attributes to evolve during a fairly straightforward unilinear evolutionary progression. While it is certainly true that nowadays researchers in the field recognize a far from straightforward evolutionary progression, if I did have one criticism of an otherwise well put together and engaging exhibition it would be that it did not convey this fact as well as it could have! 

On an information panel at the main entrance to the exhibit it is noted that the study of human origins can seem quite daunting to the non-specialist, with the state of knowledge in the field seemingly in constant flux, whether as the result of new fossil discoveries or the application of new analytical techniques made possible by technological advances. The exhibit-goer is warned that by the time the exhibit has opened it may already be out of date. Yet apart from mention of the Denisovans and of Late (or Upper) Pleistocene interbreeding between them, Neanderthals, and our own species, Homo sapiens - major discoveries since 2010 thanks to the ability to retrieve genetic material (DNA) from fossils - the major discoveries and advances in thinking of the last 25 years were largely ignored. The result was an overall impression of human evolution over the longer term that was essentially unchanged from what one might have read about in the mid 1990s (and which had largely developed back in the 1970s), when East African finds dominated the narrative. This picture had the line leading to Homo sapiens starting with Ardipithecus ramidus and progressing via the increasingly more human-like Australopithecus afarensis, early Homo sp. (habilis/rudolfensis), African Homo erectus (aka. Homo ergaster), and Homo heidelbergensis (a broad and perhaps problematic species designation that is often identified as the most recent common ancestor of Homo sapiens and Neanderthals). The southern African species Australopithecus africanus was pushed to an evolutionary 'side branch' in this scenario, along with the 'robust australopithecines' in the genus Paranthropus. 

The twenty first century discoveries in South Africa of Australopithecus sediba and Homo naledi (to name just two) have hinted at much more complexity in the story of our origins, and at the very least have forced us to take another look at the role of other parts of the African continent in that story. Australopithecus sediba may share more skeletal features with early Homo sp. than any other australopithecine species, leading to the suggestion that it could be ancestral to our genus. Of course, if we were to extend recent thinking about a pan-African origin for our species, Homo sapiens, further back in time, it could be that several populations of australopithecines in more than one region of Africa were ancestral to our genus.  

 

As noted above one analytical technique made possible by technological advances that has had a massive impact on the study of human origins is the retrieval of genetic material (DNA) from fossils! And a new study out last week in the journal Science promises to be another 'game-changer' in the field, because it turns out that we don't actually need fossils to get hominin DNA!  

Researchers at the Department of Evolutionary Genetics at the Max-Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a team of international collaborators report the successful extraction and analysis of trace amounts of nuclear DNA (nDNA) in sediments from cave sites in Spain and Siberia (including Denisova Cave, where 'Denisovans' were first identified) that showed signs of hominin occupation back to about 200,000 years ago. 

In a study published in Science in 2017 hominin mitochondrial DNA (mtDNA) was successfully recovered from sediments of a similar age. As mentioned in my previous post, this was the type of DNA recovered from sediments from Baishiya Karst Cave on the Tibetan Plateau and which confirmed that site as just the second known site where Denisovans were present. mtDNA was also the first type of Neanderthal DNA to be successfully extracted from a fossil and sequenced, more than a decade before a Neanderthal nDNA genome was sequenced. The main reason why mtDNA is typically easier to extract and analyze is that it is present in much higher copy numbers in a cell (while cells contain one copy of the nuclear genome, there are often thousands of copies of the mtDNA genome). It is therefore more likely to be preserved in archaeological remains (Matisoo-Smith & Horsburgh 2012). The big disadvantage of mtDNA, however, is that it does not provide as complete a picture of a population's genetic history as nDNA. The reason for this is that mtDNA only carries information about maternal lineages (an individual's mtDNA is inherited only from their mother, while their nDNA is inherited from both parents). To illustrate the difference this can make to our picture of the past, on the basis of the analysis of Neanderthal mtDNA only there was no evidence for interbreeding with Homo sapiens - this would only become apparent following analysis of nDNA genomes (see the last two linked papers above). The new paper therefore indicates a considerable advance in the potential information available to those researching our evolutionary past! 

Exciting times ahead!

Thanks for Reading,

Nick  

Book Reference: 

Matisoo-Smith, E. & Horsburgh, K. A. 2012. DNA for Archaeologists. Walnut Creek, CA: Left Coast Press.

(see in-text hyperlinks for original research papers)