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Posts Tagged evolution

yes, we have some bananas – just not gm ones (yet) Alison Campbell Mar 09

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Back in 2010 I wrote a post about bananas, following on from a Schol Bio question the previous year. As well as looking at the genotypes of modern bananas, I highlighted the fact that the original wild banana was not a particularly appetising object, with little flesh and a lot of large, hard seeds. Selective breeding for the win!!!

Anyway, it seems like at least one anti-GMO Facebook group has got a case of the vapours about ‘GMO bananas’, this being the only explanation some of its members could think of for the fibres that you’ll often see in the skin of a not-completely-ripe banana when you twist & pull the stalk end. Kevin Folta picked this up in a blog post, noting the lack of knowledge of some of the commenters there (I haven’t quite decided if the one about Morgellon’s is a poe…) – but as Robert Sacerich notes, one of those commenters is giving an object lesson in how NOT to do science communication, & doesn’t help the cause.

And no, we have no GM bananas – at the moment. Sacerich points out that there’s work in progress on developing GM plants that are resistant to the major threats to banana production (Black Sigatoka disease, Banana Bunchy Top virus, and bacterial infections). So such plants may well become a reality in the relatively near future.

But that will have nothing to do with the fibres that so concerned those anti-GMO commenters; they’ve always been with us & were apparently used in cloth production in Japan as early as the 1200s, a practice that’s seen something of a recent resurgence. (I didn’t know that! You learn something new every day.)

a mantis? or a fly? Alison Campbell Feb 28

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So, which is it? A mantis? Or a fly?

(Image by kind permission of Daniel Llavaneras)

In fact, the creature shown in this gorgeous image by Daniel Llavaneras is neither mantis nor true (Dipteran) fly, although its common name is ’mantisfly’. Instead, it belongs to the insect family Mantispidae (a group that includes lacewings and antlions). Like real praying mantids, matisflies walk on 4 legs, with the front pair folded as shown, and the head is somewhat mantis-like. The adults hunt as mantids do, shooting out those raptorial front legs to catch small insects, while the larval diets vary: some are also active predators, while others consume wasp & beetle larve, or spider eggs (later pupating in the spider’s egg sac). In adult form & behaviour, the mantisflies are an excellent example of convergent evolution.

a tale of two tails Alison Campbell Feb 18

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Lizards, like us, are chordates. One of the defining characteristics that all chordates share at some point in their development is the presence of a notochord: a stiff rod of tissue that runs along the dorsal side of the animal, just beneath the hollow dorsal nerve cord. (Yes, hollow. This is the result of its origins in the neural tube that forms early in chordates’ embryonic development.) In most vertebrates the notochord’s replaced by the spinal column. Another chordate feature is the presence of pharyngeal pouches (homologous to gills in fish, and to structures in the jaw and inner ear in mammals), and there’s also the tail. A tail that extends beyond the anus. And it’s that last fact that sets lizards & scorpions apart, when it comes to losing their tails.

This ability to shed the tail is known as autotomy, and it seems to have evolved in response to predator pressure: the tail may even continue to wriggle for a while, which would help to distract a carnivore long enough for the lizard to escape and to live another day.

And that longer-term survival post-autotomy has much to do with the fact that a chordate’s tail is ‘post-anal’. For when a lizard (eg a gecko, or a skink) loses its tail, the animal’s gut remains intact; it can continue to take food in at one end & pass faeces out the other.

Scorpions are arachnids, related to spiders and mites. As a paper published earlier this year in PLoS ONE notes (Mattoni et al, 2015), scientists have known about autotomy in arachnids, but up until now they’d only observed the voluntary loss of legs. However, Mattoni & his co-workers augmented data from the field, and from museum specimens, with some (very careful!) experiments on live animals to demonstrate that at least some species of scorpions are able to detatch their tails.

As for lizards, a tail (more correctly, a ‘metasoma’) continues to wriggle for a while after it’s detatched, and may also act as a distractor to allow the animal to escape a predator. There is, however, a drawback – with its tail the scorpion also loses its anus and the penultimate portion of its digestive tract. And neither metasoma nor gut regenerates.

On the face of it, you have to wonder why caudal autotomy (the ability to voluntarily shed the tail) would ever have been selected for in scorpions. They’re unable to sting ever again, which would leave them with a much-reduced ability to defend themselves or to kill large prey items. And once the open end of the intestine is closed by scar tissue – which takes about 5 days – they can no longer pass faeces from the gut, which must put a dampener on their ability to take food in at the other end – a case of enforced constipation? (The authors note that in at least some cases, the pressure of accumulating poo may trigger another autotomic event, when the animal loses the segment at the ‘new’ end of the tail.)

However, for the scorpions, all was not lost. The researchers’ lab experiments showed that the tail-less arachnids still managed to survive for up to 8 months post-amputation, occasionally eating small prey items. Which would be irrelevant if they were unable to pass their genes on – but the animals were also able to reproduce. In mating experiments, tail-less males were nonetheless able to court and mate with females on multiple occasions. This means that tail-shedding may still provide a selective advantage, in that it allows animals to escape predation and go on to reproduce.

You should also read Ed Yong’s take on how the scorpion lost its tail :)

C.I.Mattoni, S.Garcia-Hernandez, R.Botero-Trujillo, J.A.Ochoa, A.A.Ojanguren-Affilastro, R.Pinto-da-Rocha,& L.Prendini (2015) Scorpion sheds ‘tail’ to escape: consequences and implications of autotomy in scorpions (Buthidae: Ananteris) PLoS ONE. doi: 10.1371/journal.pone.0116639

engraving by homo erectus – art? or doodling? Alison Campbell Dec 11

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Why is it that practically every time there’s a new discovery relating to the evolution of our own species, there is a headline saying that this finding ‘could rewrite human history’?

Because, bingo! At least one newspaper report1, of a paper published last week in Nature, carried the header: “Homo erectus engraving could re-write human history, and might show art began 300,000 years earlier than we knew.”

Now, the story’s really interesting & surely didn’t need the overblown headline, even if one of the research team was reported as using the phrase. Certainly the work of a large team of researchers (Joordens et al, 2014) has pushed back the dates for human use of symbols, to around 0.5 million years ago (on the basis of 40Ar/39Ar and luminescence dating), which is far older than the carvings and paintings of Cro-Magnons, and perhaps Neandertals - but doesn’t necessitate a total rewrite of our history. And if their attribution of the finds to erectus is correct, then it extends our understanding of cognition in this species. (In fact, headlines like that fall right into the hands of creationists.)

This is a nice piece of detective work, & it also shows how serendipitous some discoveries can be: one of the team, Stephen Munro, noticed the marked mussel shell in photos he’d previously taken of specimens in a museum collection in Leiden, and that sparked a thorough investigation of the provenance and age of the shells. It turns out that the shell assemblage was originally collected at Trinil in Indonesia – the same location, and in fact the same strata (the ‘main bone layer’) as that of the ‘type’ specimen for H.erectus, collected in 1891 by Eugene Dubois. This led the team to the conclusion that this marked shell, and what looks like intentional damage to other shells, were the work of Homo erectus.

So what can we tell from these results? Well, it looks as if erectus enjoyed a good feed of seafood from time to time. The evidence for this lies in shells with holes in them – holes that lie over the position of the adductor muscle that holds the shell closed. Around 1/3 of the shells from this particular site had these holes, & overall the shell assembly contained “only large adult-sized specimens (about 80-120mm in length), while under normal conditions mussel populations contain all size classes” (Joordens et al, 2014): this strongly suggests that the molluscs were deliberately collected.

As for the holes themselves – the research team ruled out the possibility of damage by non-human predators, but noted that comparable holes were made in gastropod shells by pre-Hispanic modern humans living in the Caribbean. They went on to experiment on modern mussels and found that someone could use a tool such as a shark’s tooth to drill a hole in the animal’s shell over the adductor muscle; piercing the adductor caused the bivalve’s shell to open. This speaks both to erectus‘ ability to conceive of and use rather smaller tools than we usually associate with them, and to their knowledge of shellfish anatomy. (You’d certainly find molluscs opened this way much easier to eat than if you had to bash them with a rock!) Another shell appears to have been retouched using a flaker, presumably for use as a scraper or other tool.

The team then looked at the geometric lines found on the outer surface of one shell & determined that they were highly unlikely to be due to the shell knocking around with other shells & stones, but were probably produced using a shark’s tooth or something similar. The lines were most likely laid down while the shell was fresh & so covered with the coloured periostracum common to mussels, “which would have produced a striking pattern of white lines on a dark ‘canvas’ ” (ibid.) The lines are quite deep, would have required a fair bit of force and also good manual control to make, and Joordens & her colleagues concluded that “a single individual made the whole pattern in a single session with the same tool” (ibid.).

So, we’ve got evidence of what may be the earliest known use of a shell as a tool; evidence of Homo erectus including seafood in their diet; and evidence of someone consciously & deliberately scoring lines in a fresh mussel shell. But was it ‘art’? And does it really necessitate the rewriting of our entire evolutionary history?

 

1 I’m not sure why the Independent reporter correctly identified the engraved lines as being on a shell & then went on to talk about them being on ‘a rock’. Poor subbing?

 

J.C.A.Joordens, F.d’Errico, R.P.Wesselingh, S.Munro, J.de Vos, J.Wallinga, C.Ankjaergaard, T.Reimann, J.R.Wijbrans, K.F.Kuiper, H.J.Mucher, H.Coqueugniot, V.Prie, I.Joosten, B.van Os, A.S.Schulp, M.Panuel, V.van der Haas, W.Lustenhouwer, J.J.G.Reijmer & W.Roebroeks (2014) Homo erectus at Trinil on Java used shells for tool production and engraving. Nature  doi: 10.1038/nature13962, published on-line 3 December 2014

from small beauties to a big one Alison Campbell Dec 07

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Is it a peacock? Is it a turkey?

Another in the occasional series of gorgeous creatures: the ocellated turkey :)

Image credit: backyardchickens.com

Over on Tetrapod Zoology, Darren Naish provides the detailed story of this species’ biology & evolution.

Apparently they are difficult creatures to keep in captivity, so they won’t be appearing on the Christmas menu any time soon. They’re native to an area of about 130,000 square km across northern Belize, northern Guatemala, and the Yucatan peninsula.

When I first saw an image of this stunning bird (on FB, as one might expect) I thought I was looking at the male of a strongly dimorphic species. However, it turns out that both sexes share this spectacular colour pattern, although the colours may be somewhat muted in females. They’re easier to distinguish in the breeding season, because the red & yellow lumps, or nodules, that dot the head & neck swell in males & become even more brightly coloured.

Sadly, as Matt Milner notes on the Cool Green Science blog

Most conservationists consider it near-threatened, with deforestation making the birds easier to kill by local subsistence hunters, a major factor in the species’ decline. 

The North American wild turkey got pushed close to the brink of extinction in New York state & has since bounced back due to careful management of the population and it’s habitat, so there’s hope for its gorgeous cousin if suitable conservation mechanisms can be identified & put in place.

 

sticky little lizard feet Alison Campbell Oct 31

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Evolutionary change can be fast – Peter and Rosemary Grant’s long-term & ongoing research project on the Galapagos finches documented rapid responses to environmental changes, for example, as does the  recent work on cane toads in Australia. And biologists have known since Darwin’s time that competition can be a strong driver of evolutionary change. (Take Gause’s principle of competitive exclusion & its implications, for example.) A just-published paper about Anolis lizards demonstrates this very well (Stuart et al., 2014).

The way in which different species of this little lizard divvy up their habitat is used as an illustration of niche partitioning by many textbooks (you’ll find an example here). Stuart & his co-authors describe some elegant experimental work over a period of 15 years, on artificial islands in a Florida lagoon. Initially they used six of these islands, all of which were already colonised by the green native anole, Anolis carolinensis: three of the islands acted as controls, while brown anoles from Cuba (Anolis sagrei) were introduced to the other three. The two species are described as being “very similar in habitat use and ecology”, including diet, so they’d be expected to compete fairly strongly when brought together.

In other areas where the two species are found together, A.sagrei perches lower in trees than carolinensis, which left to itself would occupy most of the tree. So the prediction was that on islands where sagrei was introduced the same thing would happen: carolinensis would come to occupy a reduced niche, perching higher than the ‘invader’. And this is indeed what happened, in the space of three months:

by August 1995,on treatment islands already showed a significant perch height increase relative to controls, which was maintained through the study.

The researchers also predicted that this change in niche would be accompanied by a change in morphology; specifically, that there would be selection for larger, sticker feet in A.carolinensis, on the basis that

[toepad] area and lamella number (body-size corrected) correlate positively with perch height among anole species, and larger and better-developed toepads improve clinging ability, permitting anoles to better grasp unstable, narrow, and smooth arboreal perches.

This prediction was tested through observations on 11 islands, five with only the native species and six with both the native and the Cuban invader. Again, carolinensis perched significantly higher in trees on islands where sagrei was also present – and on those islands carolinensis anoles also had “larger toepads and more lamellae” than were found on the same species living without the competitor (an example of character displacement) – and this happened within about 20 lizard generations.

Careful analyses allowed the researchers to rule out other explanations:

In sum, alternative hypotheses of phenotypic plasticity, environmental heterogeneity, ecological sorting, nonrandom migration, and chance are not supported; our data suggest strongly that interactions with A. sagrei have led to evolution of adaptive toepad divergence in A. carolinensis.

So, just as with the cane toads, we are seeing rapid evolutionary change in real time.

Y.E.Stuart, T.S.Campbell, P.A.Hohenlohe, R.G.Reynolds, L.J.Revell & J.B.Losos (2014) Rapid evolution of a native species following invasion by a congener. Science 346 (6208): 463-466. doi: 10.1126/science.1257008 

rapid evolution in cane toads Alison Campbell Oct 27

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In her book Paleofantasy, Marlene Zuk discusses cane toads (Bufo marinus) as an example of just how rapidly evolutionary processes can work. These amphibian pests were introduced into Australia in 1935 to control borer beetles in sugar cane. Unfortunately the toads never got the memo about this expectation, and have spread rapidly across the continent, damaging a range of native ecosystems as they go. (They’re aided by the fact that they’re toxic, killing many of the predatory animals that might otherwise eat them.)

And it’s not just that the toads are and always have been fast hoppers. As this article says

When the toads were first introduced, they spread at a rate of about six miles (ten kilometers) per year. Today cane toads advance more than 31 miles (50 kilometers) annually.

In other words, they’re getting faster, with animals at the ‘invasion front’ moving up to 1.8km in a night. (The researchers were able to measure the toads’ speed by fitting them with miniature radiotransmitters, strapped to their waists.) Phillips & his colleagues (2006) point out that speed of movement in toads is correlated with leg length, and asked the question: is there a difference in average leg length between toads at the front of the amphibian wave spreading across Australia, and those at the back of the bunch? The answer:

As the toad invasion front passed our study site, we measured relative leg lengths of all toads encountered over a 10-month period. Longer-legged toads were the first to pass through, followed by shorter-legged conspecifics (order of arrival versus relative leg length: r = -0.34, n =552, P = 0.0001). Longer-legged toads therefore moved faster through the landscape.

And the evolutionary changes don’t stop there. In a paper just out, Brown, Phillips & Shine (2014) describe how the animals’ tendency to travel in a straight line has changed too:

Radio-tracking of field-collected toads at a single site showed that path straightness steadily decreased over the first 10 years post-invasion.

The research team found that this behavioural change had a genetic underpinning. The progeny of toads from the invasion front moved in straighter paths than the offspring of toads from older, well-established populations to the east. In addition, “offspring exhibited similar path straightness to their parents.” Brown & his colleagues concluded that

The dramatic acceleration of the cane toad invasion through tropical Australia has been driven, in part, by the evolution of a behavioural tendency towards dispersing in a straight line.

G.P.Brown, B.L.Phillips & R.Shine (2014) The straight and narrow path: the evolution of straight-line dispersal at a cane toad invasion front. Proc.R.Soc. B 281(1795) doi: 10.1098/rsph.2014.1385

B.L.Phillips, G.P.Brown, J.K.Webb & R.Shine (2006) Invasion and the evolution of speed in toads. Nature 439: 803. doi: 10.1038/439803a

Teachers: there’s an open-access summary of the 2006 paper here.

‘paleo’ diet? or paleofantasy? Alison Campbell Oct 17

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The ‘paleo’ diet story on Campbell Live tonight spurred me to finish my review of one of the most entertaining popular books on genetics that I have read for some time. Entertaining, and informative, in equal measure. I wonder what author Marlene Zuk would have made of the TV story.

book cover Marlene Zuk (2013) Paleofantasy: what evolution really tells us about sex,diet, and how we live.  Norton (New York)

ISBN 978-0-393-34792-0 (paperback)

For in that story we heard gems like this: “It’s a commitment to eating food that is unadulterated, eating food in its most natural state.” Paleo proponents (says the TV story) believe our most natural diet is that of our Palaeolithic cavemen ancestors. Somehow I doubt our ‘cavemen’ ancestors were eating avocados, beetroot, bacon or kale. (There’s also an air of chemophobia, with one proponent of paleo eating stating that their diet contains “[n]othing nasty and nothing you can’t pronounce” – which reminded me of the series of posters by Australian teacher James Kennedy, showing the list of chemical compounds found in natural food items: blueberries, anyone?).

Proponents of the so-called paleo diet seem to think that humans haven’t evolved in the last 10,000 years (since the advent of agriculture), and that this means that our bodies aren’t ‘designed’ to cope with the products of the agricultural revolution. (This, while eating foods that bear little resemblance to their Palaeolithic counterparts. Look at teosinte, the ancestor of maize, for example: small, stone-hard kernels arranged in a few lines on a stalk. Nothing like the fat, soft, juicy kernels on a modern cob of corn.)

As Zuk notes, the paleofantasy happily assumes that at some point in the past (around 10,000 to 40,000 years ago, depending on who you’re listening to), humans were perfectly adapted to their environment, including their diet. But, she asks, why hark back to that particular point in time?

would our cave-dwelling forebears have felt nostalga for the days before they were bipedal, when life was good and the trees were a comfort zone?

Plus, of course, there’s the question of just which ’cavemen’ we’re aspiring to be like. We’ve no guarantee that the life-styles of modern hunter-gatherer populations are a good approximation of life 40,000 years ago. Should we be Inuit, or Kung?

And there’s no reason for us to have stopped adapting to evolutionary pressures once agriculture became the mainstay of human populations – in fact, there’s a great deal of evidence to the contrary, some of which I’ve written about previously -the evolution of lactase tolerance, for example. Similarly, with the spread of arable farming, those with the ability to digest grains would be at an advantage, to the extent that there is a higher number of copies of the gene coding for salivary amylase in populations with a long history of eating starchy grains, compared to populations where the diet has traditionally been low in starches. And Zuk provides many examples of just how rapid evolutionary change can be, in humans and in other animals (changes in cane toad morphology, in the short span of time since their arrival in Australia, are a particularly elegant case in point). The final chapter, which gives considerable detail in answering the question, are we still evolving, would be very useful to biology teachers during human evolution classes.

In other words,

[t]he notion that humans got to a point in evolutionary history when their bodies were somehow in sync with the environment, and that some time later we went astray from those roots – whether because of the advent of agriculture, the invention of the bow and arrow, or the availability of the hamburger – reflects a misunderstanding of evolution.

As the extended title of her book points out, Zuk feels that the paleofantasy extends well beyond the current diet fad. It influences beliefs about health and illness, about family life, about sex. (This last is the focus of all sorts of wistful imaginings: the book provides an entertaining sample of these.) Do bonobos, for example, really provide a good model for how human sexual activity might have been before modern mores took over? I can’t see it myself: humans and their chimpanzee cousins have follwowed separate evolutionary trajectories for 5-6 million years, and there’s no good reason why either species should closely resemble the last common ancestor. And that goes for aspects of intimate morphology as much as for behaviour: I did not know that chimpanzees have penis spines –  ”hardened growths that may serve to sweep away the sperm of previous mates.”

Zuk concludes that the paleofantasy is just that, a fairy tale – and one that limits our understanding of our own biology and evolutionary history:

But to assume that we evolved until we reached a particular point and now are unlikely to change for the rest of history, or to view ourselves as relics hampered by a self-inflicted mismatch between our environment and our genes, is to miss out on some of the most exciting new developments in evolutionary biology.

 

Anyone interested in hearing Professor Zuk speak should check out the details of her upcoming lecture tour. I’ll be grabbing a ticket to the Hamilton event!

 

fluffy the dinosaur Alison Campbell Aug 11

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Over the last 20 years quite a bit of evidence has accumulated indicating that at least some dinosaurs were feathered, much of it in the form of beautiful fossils from China. Up until now all the feathery dinos have been members of the carnivorous theropods, but this new paper by Godefroit et al (2014) extends that fluffiness in its description of a herbivorous dinosaur, Kulindadromeus zabakialicus. (The full paper is behind a paywall but the BBC offers a good general summary.)

It’s now generally accepted that birds evolved from a theropod lineage (Michael Benton discusses the evolutionary changes that this entailed, here), although there is still debate around the origins of things like wings, feathers, and when birds/dinos first took to the air. Most people are probably familiar with at least the name of Archeopteryx, but since 1994 those Chinese fossils have shown us that many more theropods were feathered, and that feathers evolved well before the first bird-like creatures took to the air. Godefroit & his colleagues comment that

fully birdlike feathers orginated within Theropoda at least 50 million years before Archaeopteryx.

and there’s even discussion around whether the fearsome T.rex may have been feathery/fuzzy.

But Kulindadromeus wasn’t a theropod – it was a ‘neornithischian’ – an early member of the ‘bird-hipped’ dinosaurs, a group that includes Stegosaurus and Triceratops. (This nomenclature can get a bit confusing, especially when you consider that birds evolved from ‘saurischian‘, or ‘lizard-hipped’ dinos.) And while it didn’t have the sort of feathers that we’re familiar with today, it did have a range of other structures in addition to the usual scales:

monofilaments around the head and the thorax, and more complex featherlike structures around the humerus [upper forelimb], the femur [thigh], and the tibia [lower leg].

It’s early days yet. But if other ornithischians are found with  feathers, then then this would raise the possibility that the common ancestor of both dino groups also had some sort of feathery structures on its body, and would support the authors’ suggestion that

feathers may thus have been present in the earliest dinosaurs.

In other words, feathers may well be much, much older than we’ve thought.

 

P.Godefroit, S.M.Sinitsa, D.Dhouailly, Y.L.Bolotsky, A.V.Sizov, M.E.McNamaram M.J.Benton & P.Spagna (2014) A Jurassic ornithischian dinosaur from Siberia with both feathers and scales. Science 345: 451-455 . doi: 1126/science.1253351

human facial features the result of being used as a punching bag? somehow I don’t think so Alison Campbell Jun 11

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I saw this story in the newspaper yesterday, & again today on one of the science feeds:

Researchers in the US have studied the skulls of ancient human ancestors and concluded that fist-fighting may have played a role in shaping the male face.

You can read the paper itself here (Carrier & Morgan, 2014). I’m sorry, but to me it reads like a just-so story. Just because modern humans take a swing at each other from time to time, doesn’t mean that this was the case for earlier hominins. The authors of the paper argue that the facial features of robust australopithecines are the result of natural selection acting through bare-fist fighting. However, they don’t offer any actual evidence that this might have happened: nothing on whether paranthropine skulls show the sort of facial damage that you might expect if fighting in this way was sufficiently widespread to act as a selective force. And similarly, no real discussion of whether Paranthropus could form a fist capable of doing such damage. (The paper on Australopithecus sediba to which they refer actually describes sediba‘s hand as a mosaic of features.) In other words, they’re making a sweeping assumption – that paranthropines routinely beat the heck out of each other – to support the a priori assumption that our own facial evolution was shaped by this.

There’s also the question of whether modern human faces show much evidence of having evolved in this way; they actually seem quite prone to damage. Noses & cheekbones are rather susceptible to damage, and the bones of the cranium – thinner than those of Paranthropus - are dangerously easy to break. At the same time, according to the authors’ speculative view, our hands are particularly well adapted to deliver blunt-force trauma.

This quote from the paper (emphasis mine) says it for me; we really are dealing with conjecture & imagination:

Starting with the hand of an arboreal great ape ancestor, it is possible to imagine a number of evolutionary transformations that would have resulted in a club-like structure adapted for fighting.

Rudyard Kipling might have appreciated it – a point also made by Brian Switek in his excellent commentary over at National Geographic.

Carrier, D. & Morgan, M. (2014) Protective buttressing of the hominin face. Biological Reviews doi: 10/1111/brv.12112

 

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