A Mini-Post & Anthro News

Hominin herpes, European apes, and a fossilized spine

Sometimes a lot of cool stuff happens between BS&M meetings. In an effort to keep up with the constant flow of science and to tide you over until our next discussion, we’re going to try to post mid-week mini-blogs and links to what we’re reading during the week.

This week, Google alerted me to another instance of possible between-species hanky-panky in the fossil record. In a new analysis, Underdown and colleagues attempted to figure out the most likely pathway through which humans got genital herpes (HSV2) from the ancestors of chimpanzees. Yes, you read that right. The closest relative of human HSV2 is not HSV1 (oral herpes), but ChHV1 (the chimpanzee version of herpes). The authors suggest that these two viral lineages split from one another between 1.4 and 3 million years ago, and that either Homo habilis got “proto-HSV2” from the ancestor of modern chimps and gave it to Paranthropus boisei, who then passed it on to Homo erectus, or P. boisei got it directly from the ancestor of modern chimps and transmitted it to H. erectus. (H. erectus is generally considered directly ancestral to Homo sapiens, which is why the virus only has to make it to that species to end up in us.)

Before things get too weird, I want to point out that the authors don’t think that the interspecific hanky-panky went down between either H. habilis or P. boisei and a member of the population of ancestral chimps. They suggest that hunting or scavenging meat from infected chimpanzees would have likely been enough to pass the virus on to one of the hominins, probably via chimp blood coming into contact with an open wound during the butchery process. Once “proto-HSV2” made it into H. habilis or P. boisei, however…

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Anyway. HSV2 now joins HPV (from Neanderthals) and body lice (from some archaic form of Homo) as evidence of ~close~ contact between humans and our hominin cousins (Reed et al. 2004, Pimenoff et al. 2017). The coolest thing about all of this research is that it’s not based directly on fossils or on ancient DNA; you can use things like the evolution of viruses to tell us about our own evolution. Awesome.

Read on for links to what we’ve been nerding out over this week and the references for the herpes paper.
Note: the featured photo is the OH5 cranium of Paranthropus boisei (credit: efossils.org)

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BS&M Does Homo floresiensis

Ghost Lineages – So Hot Right Now

This past Friday, our journal club took on “The affinities of Homo floresiensis based on phylogenetic analyses of cranial, dental, and postcranial characters” (Argue et al. 2017). Essentially, Argue and colleagues attempted to figure out what other hominin species H. floresiensis (often called the Hobbit) was most closely related to, using statistical tree-building methods.

Since it was published in 2004 by Brown et al., H. floresiensis has been a bit of a mystery. Much like Homo naledi, there’s been a lot of discussion about where it belongs in the human family tree because its anatomy was A) weird and B) totally unexpected for its age (somewhere between 100-60 thousand years old) and its geographic location (on Flores, a small Indonesian island). The Hobbit was very short in stature, with a very small brain (in the range of orangutans, chimpanzees, and the much-older australopithecines), large teeth for its size, primitive-looking wrist bones, and disproportionately large feet relative to its height and leg length (hence its nickname of the Hobbit). Its discovery on Flores was a surprise because the other hominins that have been found in Indonesia (like Homo erectus from Java) were older and had larger brains (and we generally think brain size in the human lineage has increased over time – but last week’s chat about H. naledi also brought this up as a problematic assumption).

In their new article, Argue et al. set out to test two hypotheses: either the Hobbit is a late survivor from an earlier primitive hominin lineage, or it is a dwarfed descendent of H. erectus. They also commented on another controversial claim that’s been made about the Hobbit – that it is simply a modern human with a genetic/developmental pathology. They tested their hypotheses by applying two tree-building methods to a large sample of characters (particular features or measurements of the skeleton) from the skull, teeth, and postcranial (below the head) skeleton. One method (parsimony) attempts to build the shortest possible tree (one that requires the fewest changes in traits to get from species to species), while the other method uses probability to figure out which trees are most likely to occur (given a particular model of evolution).

When you set out to do a project like this, you’re forced to make some choices as a paleoanthropologist. If you have isolated postcranial bones from a hominin site where you’ve previously found fossils of more than one hominin species from the same time, how do you decide which body belongs with which head? You also confront the issue that not all researchers agree on which specimens belong in which species. And, as always, the fossil record is incomplete; you don’t have all of the characters for all of the species. To account for these potential problems, Argue et al. tested their two hypotheses with several different data sets – for example, they did one test where they considered all of the potential postcranial skeletal material that’s been called Homo habilis to actually be H. habilis and another where they excluded the questionable material.

What Argue et al. found was that their two different hypothesis testing methods and various different data sets produced broadly similar results in support of the first hypothesis: the Hobbit either shared a common ancestor with Homo habilis or is the sister group to the grouping of Homo habilis/Homo erectus/Homo ergaster/Homo sapiens. They are able to reject the hypothesis that the Hobbit is a dwarfed H. erectus (and reject a number of other species as possible close relatives). What this suggests is that (as was proposed for Homo naledi in last week’s papers) there is a long ghost lineage (unknown ancestors) for the Hobbit dating back more than 1.75 million years that is still waiting to be found. Ghost lineages – so hot right now.

Read on for some BS&M discussion bits!

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BS&M Does Homo naledi

On May 9th, Lee Berger and colleagues published three new papers on Homo naledi, the most recent South African hominin fossil find to make media waves. The original H. naledi fossil material was discovered in 2013 by two cavers working with Berger; it comes from the Dinaledi Chamber of the Rising Star cave system, from which it takes its species name (Berger et al. 2015). The first H. naledi discovery was remarkable because there are at least 15 individuals (from juveniles to adults of both sexes) represented in the assemblage and there are often multiple copies of each skeletal element present, which allows paleoanthropologists to look at variation within the species, and to see how it grew and developed. In total, there are about 1550 hominin bones and teeth in the assemblage – the largest single species assemblage found anywhere in Africa (Berger et al. 2015).

The three new articles covered the discovery of additional skeletal material (Hawks et al. 2017) and the age of the fossils (which had been a major source of speculation) (Dirks et al. 2017), and proposed a hypothesis for understanding Pleistocene hominin diversity in subequatorial Africa as part of a larger mammalian biogeographic pattern (Berger et al. 2017).

The new fossil material comes from a second chamber within the Rising Star system, the Lesedi Chamber, and represents at least three individuals (though in actuality the material likely comes from more than three individuals, based on where the bones were recovered from within the Chamber). The most spectacular of these remains is a relatively complete skull with associated skeletal material; the researchers have named this individual Neo. The new material looks a lot like the previously discovered H. naledi bones and teeth, and also includes both adult and juvenile material. The major thing that differentiates the Lesedi Chamber finds from the Dinaledi Chamber finds is that the Lesedi Chamber also contains animal skeletal material (Hawks et al. 2017).

The paper on the age of the fossils used several different methods (including dating geological features of the cave itself as well as directly dating some of the fossil teeth) to produce an age range for the material of 236,000-335,000 years old (Dirks et al. 2017). This means that the material is later Middle Pleistocene in age, much younger than would have been predicted based on looking at features of the skull and skeleton (like brain size).

In a previous paper, Hawks and Berger (2016) discussed what three different hypothesized ages (including a scenario that does match the new date) for the original H. naledi material would mean for its place in human evolution, and they follow up on this in the third new paper – now that we have a date, what does it mean? The date is younger than the first appearance of Homo erectus around 1.8 million years ago. As H. erectus is generally thought to be part of the lineage that is directly ancestral to us, this would seem to preclude H. naledi as a member of our direct line, unless it represents a sister group to our species that preserves a lot of the primitive morphology of a shared ancestor. This interpretation bumps H. erectus to a side branch of our family tree. A different type of analysis of the features of various species of Homo suggests instead a more bushy view of earliest Homo – whatever that ancestor was split into a number of species, each having only some of the ancestor’s features. What we can say is that H. naledi is likely only part of a branch that originated earlier in time, with the authors going so far as to suggest that we might already have fossils from earlier on this branch that we have not recognized due to their fragmentary nature (Berger et al. 2017).

Regardless of where Homo naledi ends up on our family tree, it’s still an incredible discovery that contributes to our understanding of the human fossil record. It provides a window into a time period in our history from which we don’t have great data and underscores two important ideas: first, that for most of our evolutionary history, Homo sapiens was not the only hominin species on the planet; and second, that there are still spectacular fossils waiting to be discovered.

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Welcome to BS&M

Hello! Welcome to the Bones, Stones, and Monkeys (BS&M) journal club blog. Every Friday, graduate students (and sometimes faculty) from the evolutionary side of the Department of Anthropology at Rutgers University meet up to discuss a newly published paper in the field of biological/evolutionary anthropology. While BS&M has been around for a while, we recently decided to start this digital experiment as an exercise in science communication.

What you can expect from us here is a mostly jargon-free summary of whatever paper(s) struck our fancy that week, along with some sort of commentary. Right now, we’re trying out various written formats (partial transcripts, interesting insights/questions, etc), but we have some designs on turning our anthropological shenanigans into a podcast (because, somehow, the hilarity of shouting that Homo naledi are mole people! just doesn’t come across in writing).

Hopefully you’ll enjoy our little experiment! You can also find us on Facebook at the Center for Human Evolutionary Studies at Rutgers University or on Twitter @CHES_Rutgers.