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My top 10 picks for research dealing broadly with human evolution, which I categorise broadly from primates all the way to cultural evolution in Homo sapiens. I only call it “human evolution” because that gets people’s attention. The master list contains 29 papers. [OA] indicates open access papers.
10. Evolutionary Development in Australopithecus africanus.
This paper studies ontogeny – development from child to adult – in Australopithecus africanus (the Taung child). McNulty finds that the species exhibits paedomorphism: the adult resembles the juvenile of the ancestral species. Specifically in this case, the adult resembles juvenile chimps, because the growth rate decreases after the first molar erupts – a fairly early time. The significance here is that it just provides more data points for the role of paedmorphosis and other forms of heterochrony (messing around with the timing of development) in the evolution of hominins.
9. Extremely Rare Interbreeding Events Can Explain Neanderthal DNA in Living Humans. [OA]
One of the most significant findings of the Neanderthal genome sequencing project was that there was Neanderthal DNA in humans, explainable only by saying that humans and Neanderthals interbred (specifically, in the Middle East, before the consequent spreading into Eurasia). The significance, as far as I’m concerned, is only for species concepts in hominins (how can Neanderthals be a separate species if interbreeding was possible?), but it also allows for many interesting questions: how do we explain the presence of 1-4% Neanderthal DNA in non-African humans? Why isn’t there more? How frequent and common was the interbreeding? This paper attempts to answer these with a probabilistic population genetics model in which humans and Neanderthals are equally fit and coexisting (if that assumption bothers you, it actually squares up with recent research into Neanderthal biology, so it’s warranted). The result is that successful interbreeding was quite rare: 1 pair every 77 generation is enough, although more are allowed by the model with slightly less probability. The rarity of the events is either due to biological reasons (sterile hybrids) or, more interestingly, due to cultural segregation.
Speaking of Neanderthals and humans, Volcanic ash layers illuminate the resilience of Neanderthals and early modern humans to natural hazards shows that they both weren’t much bothered by natural disasters.
8. Evolutionary morphology, cranial biomechanics and the origins of tarsiers and anthropoids.
This is a detailed paper on the tarsier fossil record, and what it says about the evolution of tarsiers and especially their skull. It’s significant to study these because tarsiers are pretty close to the anthropoids (the group of primates that includes great apes and monkeys, i.e. humans as well), either as a sister group or as a final remnant of a larger and now mostly extinct clade. The authors support the latter hypothesis, but do admit that the fossil record is currently too sparse to support either view conclusively. Their most important evidence is in the skulls. Tarsiers, as the drawing above shows, have huge eyeballs – the largest relative eyeball size in all mammals. Associated with this is a highly-specialised skull to support the eyeballs and their knocking about as the tarsier leaps around trees. Fossil tarsiiforms also have a similar skull, therefore modern tarsiers must be nested within that group. Otherwise, they would have had to have evolved convergently, which just isn’t parsimonious. However, it’s also true that the body fossil record of these tarsiiforms doesn’t show that they were leaping animals like modern tarsiers. Read the paper for much more in-depth discussion – it also serves as an excellent review paper, including on the history of research into tarsier origins. It also has interesting things to say about how phylogenetics should work – not by calculation, but by functional and ecological consideration. And that’s a view I sympathise with (I try to incorporate both in my own research).
7. U-Series Dating of Paleolithic Art in 11 Caves in Spain.
In this paper, Pike et al. present their absolute dating of cave art Spain. The difference from previous studies is that previous studies have not dated the art itself, but the calcite that covers it, which introduces uncertainties into the dating; this study dates the actual artwork directly. The most significant find is a pushing of the oldest cave art by 4000 years: the prvious record was in Grotte Chauvet, France, at 35-39 ka. The new record is from El Castillo at 40.8 ka. The other significant finding is the confirmation that there is indeed an increase in artistic complexity over time, both visually and spiritually.
Such increasing complexity is emblematic of human culture. In Identification of the Social and Cognitive Processes Underlying Human Cumulative Culture, Dean et al. show how humans differ from chimpanzees and capuchins in their cultural transmission methods, allowing future generations to build on the insights of previous humans. It all has to do with our enhanced social cognition, with the four keywords being: pedagogy, communication, imitation, prosociality.
6. Late Middle Eocene primate from Myanmar and the initial anthropoid colonization of Africa.
This paper describes the teeth of a new species, Afrasia djijidae, from the late Middle Eocene of Myanmar. The authors conclude that it’s the sister taxon to the contemporaneous Afrotarsius libycus from Libya. If this isn’t the result of convergence, then this gives us a precise timing for at least one dispersal of stem anthropoids into Africa: an Afrasia population migrated across the Tethys to Africa and radiated. It’s very likely that this was one of many such dispersal events however, so don’t go around treating Afrasia as your n(great)-grandfather – there could well have been other stem anthropoid dispersals, if phylogenies are to be trusted (they are).
5. Insights into hominid evolution from the gorilla genome sequence. [OA]
The biggest surprise here is that gorillas are really, really close to humans. I would be interested in seeing a brand new thorough phylogenetic study of African great apes, taking phylogenomics (all African great apes are now sequenced) and morphology into account. In any case, there’s a lot of information in this paper, and it’s open access, so go ahead and read it. The key points to take away are that there is a lot of parallel genetic evolution that took place in gorillas, humans, and chimps; and gorillas split away from chimps and humans 10 million years ago.
4. New fossils from Koobi Fora in northern Kenya confirm taxonomic diversity in early Homo.
This paper describes two lower jaws and a face discovered in 1.78-1.95 Ma deposits in Kenya. I can’t go into the details of them in any comprehensible way, suffice it to say that they’re only 2 new Homo species.
With the addition of more Homo species on the basis of only morphology, there are always detractors that find the evidence tenuous and would rather have larger, more inclusive species (they’re called lumpers, as opposed to splitters who like many species). In Unexpectedly many extinct hominins, phylogenetic methods are applied to estimate how many extinct species should be expected from the hominin fossil record. 8 is the optimal number, but up to 27 can be accomodated. So, split away.
For an in-depth look at how splitters and lumpers debate, check out The status of Homo heidelbergensis (Schoetensack 1908) [OA], which goes through the arguments proposed for making H. heidelbergensis a new species.
3. The diet of Australopithecus sediba.
Those of you who follow human evolution remember the huge splash Australopithecus sediba made back in 2010, with 5 papers describing it and its surroundings in one issue of Science. Add this paper to those ones: it describes what A. sediba ate, on the basis of dental wear, isotopes, and preserved microfossils on the teeth. They were herbivores, feeding on leaves and fruit, much like chimpanzees.
2. Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution. [OA]
Newborn ape skulls have a hole called the anterior fontanelle (F above). In chimps, this hole closes very early (before eruption of the first molars), while in human newborns it happens after (well, in most humans anyway). The closing is done by the fusion of the metopic sutures (M above)in the skull. The paper says is that this delay in the fusion was a key characteristic that allowed humans to expand their brain size, for three possible reasons: the growth of the frontal neocortex required changes in the skull structure and its organisation, leading to the delayed fusion; the hole gives the head some squishiness, allowing the head to fit through the very tight birth canal; the very high growth rate of the brain demands an expansive skull, provided by the fontanelle. However, it’s not a settled issue, because there are still several questions and holes (heh) in the hypothesis. Most importantly, there is no evidence showing that australopiths, the early hominids used in this study as evidence for the hypothesis, even had a tight trip through the birth canal. If it’s roomy, then their fontanelle is pretty much useless and the hypothesis isn’t very solid. The second issue is that we don’t know whether the fontanelle really is that important. Can we give birth naturally to non-fontanelled babies? If we can, again, these adaptive hypotheses aren’t very good. Third of all, the fusion of the metopic suture may not even be “predetermined”, and may in fact just arise naturally through biomechanics (chewing especially puts pressure on the skull and may lead to the fusion in the newborn). Again, the lack of an inheritable pathway puts the adaptive hypotheses on shaky ground.
1. A new hominin foot from Ethiopia shows multiple Pliocene bipedal adaptations.
The popular narrative of chimp-like ancestors coming down from trees, going out into the savannah and becoming humans is increasingly being shown to be oversimplified to the point of wrongness (and I’ll be damned if creationists quote-mine me there). The foot fossils described here add to the complexity of the story. They’re eight bones from a right foot, from a 3.2 – 3.8 Ma locality in Ethiopia. The graph above shows why the foot is significant: it’s the black star, and clusters neatly with humans and gorillas – the non-tree dwelling apes. Anatomical examination shows that the foot is equally adapted to grasping tree branches and to being walked on – in other words, the species that had it was at least facultatively bipedal. What this means in the larger context of human evolution is that bipedalism isn’t something special – it can, and has, evolved convergently.
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