Monday, April 26, 2010

With great power there must also come great responsibility

You’ve probably already forgotten about A. sediba.

After all, it’s been just over two weeks since the two new gorgeous partial skeletons from Malapa, South Africa were unveiled in Science magazine. And, after all, you’re probably a normal person who can forget about extinct hominins between their media splashes.

But there are some people who cannot.

They are a dedicated brother- and sisterhood of scientists who work around the clock on our fossil record. Adding to it, maintaining it, and interpreting it. It’s only because of this fearless gang of fieldworkers and phylogeneticists that you can live a relatively luxurious, paleoanthropology-free lifestyle.

Really. They never stop working for you.

In fact, just after the new species was announced, paleoanthropology’s two most important conferences (the annual meetings of the Paleoanthropology Society and the American Association of Physical Anthropologists) were held in St. Louis and in Albuquerque just so that hundreds of us could put our brains together, meld our minds, and figure out just what to make of these bones. If you say it’s just a coincidence that these conferences occurred mere days after the Science publications, well then you just don’t understand the extent of paleoanthropology’s superpowers.

Both Lee Berger and Darryl de Ruiter gave spectacular back-to-back presentations at the Paleoanthropology Society meetings on the Malapa fossils. Afterwards, the floor was opened up for questions. Because I am a real blogger and am not just using it as a convenient cover for another more mysterious occupation, I raised my hand, “How do you know the arms are long?”*

From the gist of Berger’s answer and from what it says in the paper, although the drawing shows the arms to be long, the actual length of the arms relative to body size is not a shut case. However, with the additional remains from more individuals still lying there, in the ground, just waiting their turns to be excavated…. Good estimates of body and limb proportions are tantalizingly close!

Not only that, but word on the street is the preservation is so tremendous that there may be organic remains beyond bones and teeth—the actual squishy parts like hominin skin and hair and who knows what else? If this is true, start bracing yourselves for a paleoanthropology story that will eclipse the hobbit! In fact, by all accounts, Berger and his team are at the site right now—while we’re just going about our normal daily lives—working hard to recover all the additional remains.

Among paleoanthropologists, there are many questions about the Malapa remains. Some question the dating (could be younger than they say!), but the general buzz seems to center on the genus. Why Australopithecus and not Homo or not something else?

An equally valid question is who cares? After all, Mother Nature wouldn’t know a genus if it smacked her in the face. Genus is important to humans alone. The genus that we use to label the Malapa specimens stands for how we interpret them. If they were Homo, they’d have to be more cognitively complex than australopiths, making stone tools, eating a more diverse diet that included meat from scavenging and hunting, walking and running with completely bipedal bodies, etc…. But as australopiths, with their tiny brains and mosaic of bipedal and arboreal (or remnant arboreal) morphology, they’re one step behind Homo—even if they’re 1.9 mya or younger and even if they overlap in space and time with early Homo.

Get ready for a complete overhaul of how the Plio-Pleistocene transition in the hominin fossil record is interpreted. Using standards that were implemented back in the 1960s when the fossil record made much more sense (because there were so many gaps!) is no longer appropriate let alone feasible.

What is easily forgotten in exciting times like these is that each new fossil that paleoanthropologists uncover will look different from all others. That’s because of variation due to sexual reproduction among other things. So the notion that something is special because it looks like nothing else we’ve ever seen before is misleading. Malapa is certainly special but clones of previously known hominins? Now THAT would be special. The trick is to know just how different something is, if it’s just normal variation within a species (with some change through time accounted for) or if it’s too much variation for within one species. If it’s the latter, then it’s a new species.

In this case, the authors have decided that the Malapa remains are too different from A. africanus and from anything else on record. And so a new legend is born and a new name, A. sediba, is added to the roster of those who make the world of paleoanthropology the most exciting realm of all!

Probably an even bigger to-do than the scientific reports on Malapa was the openness of the researchers. They brought casts of all the material and laid them out for everyone to see, plus they offered to make casts for anyone interested. Having just discovered the fossils in late 2008, this is a laudable strategy for moving paleoanthropology in a productive direction. If people are going to keep finding all these rewriting-the-textbook fossils, then getting them out quickly for the rest of the gang to see, by Berger’s team’s example, may be standard practice from here on out.

And on that note, I leave you with this final thought.

Whenever you or someone you love gets excited about a hominin fossil, whenever you fall asleep at night assured that your hominin ancestors are being respectfully reconstructed, whenever you walk safely down the street knowing that your evolutionary tree is growing properly, don’t forget to thank your friendly neighborhood paleoanthropologist.

This superhero-themed post is dedicated to my student Samantha Davis who presented a terrific poster on early Homo molar shape outline at the Paleoanthropology meetings. If only we’d had Malapa to include in the analysis, right?

*What I should have asked when I briefly had the floor back in that ballroom was, “Can we eat it?” After all (according to my cab driver from the airport who just finished Voyage of the Beagle), until recently, when scientists found a new species that was their first question. Then if they couldn’t eat it they had to figure out what else to do with it.


Anne Buchanan said...

Great post, Holly! My favorite lines, among many: "Malapa is certainly special but clones of previously known hominins? Now THAT would be special."

Ken Weiss said...

This is another great, timely, and knowledgeable post, Holly! Thanks very much.

It's interesting to compare the variation we see in primate or perhaps all vertebrate fossils, with the conservation we discussed in some recent posts. Are vertebrates more variable, and if so, why?

Also, since millions of years often separate these isolated fossils, of course they will differ, as you say. And also as you say, the 'names' are ours, not theirs.

But is there anything particular to be surprised about with these fossils any longer? Aren't they just filling in space with various kinds of variation, as we'd expect, rather than giving us any fundamentally new picture? Whether the feet were longer than the eyebrow, don't we now have the basic picture?

I mean, we don't find any specimens with, say 3 arms or 6 digits, large canines or sharp claws, etc.

If the latter is true, then the finds are of course still interesting, but is it time to move beyond "Gee-Wow!"? Except, of course, it sells magazines and television.

Ken Weiss said...

Unless, of course, they really _would_ be good to barbecue on a nice summer afternoon.....

JKW said...

The preservation is so great that there are squishy parts?! The obvious question (which connects to a previous post) is are there lips? :-)

Holly Dunsworth said...

JKW... if there are lips at Malapa I will absolutely die. The Mermaid's Tale should get first scoop on that, right?

Holly Dunsworth said...

1. No doubt A. sediba was delicious.
2. It's important to know if there was more than one branch of bipedal hominins so that's the biggest cause of the "gee-wow" with all new finds that fill in gaps.

Ken Weiss said...

Telling how many branches is rather problematic (as, for example, Jeff Long's recent work apparently shows about some assertions related to Neandertals).

And at least we should realize that genetically they would have been as alike as, say Eskimos, Fuegians, and Khoisan,etc., no?

As to Mermaid getting scoops, it's frustrating, since we've not yet started selling full-screen ads. We better hurry....

Holly Dunsworth said...

Well, just to let you in on a little secret... there is a very low rumble that dare not be spoken too loudly and it's got something to do with bipedalism being older than the LCA (knucklewalking would be derived in comparison) and if that's the case, then there could be very old bipedal lineages. But you didn't hear that from me. Ask Ardi.

Ken Weiss said...

I'll believe that when I see real, convincing evidence. That would be a problem given the consistently closer genomic hu-ch branch relative to gorillas. Possible, of course, but a kind of reversal from other closely related primates, no?

Also would that not imply a 'regression' to other apparently shared traits of primates (big canines, smaller brains and all that). Or would there have been a dull-toothed, somewhat sharp-witted, thumb-sucking, beast around, and its revolted relatives decided to go back to the ways of their ancestors?

Holly Dunsworth said...

By all recent accounts (Tracy Kivell's work, etc...) gorillas and chimps evolved knucklewalking independently, so it's not necessarily a problem there.

Ken Weiss said...

Knuckle-walking seems so specific that it's hard to see independent evolution....except that anyone who has played with their toddlers or pets probably gets down on all fours by knuckle-walking. At least, I know I did (yes, I know I'm primative!). So maybe we never actually gave it up entirely--maybe it's just 'natural' in some sense?

Holly Dunsworth said...

Was "primative" intentional? If not, doesn't matter... it's a great word! I'm primative too.

Ken Weiss said...

Yes, it was intentional. Otherwise, I could have claimed to be primal, primeval, primate, or even prime number. It's interesting that primates must have been named because somebody thought they were what God made first?

occamseraser said...

Parallel evolution of KW in Pan and Gorilla is a tough sell, and for good reasons. The Kivell and Schmitt hypothesis of parallel evolution has already been challenged:
"Morphological integration and the evolution of knuckle-walking," by SA Williams (J. Human Evol. (2010,in press and available doi:10.1016/j.jhevol.2010.03.005

With respect to Au. sediba's "long arms", the FOREARM is indeed long by any human standard; with a brachial index of ~88, it overlaps at the lower end with chimps and bonobos. The femora they published are much too fragmentary to get a reliable humerofemoral index, but the upper limb joints are large relative to the lower ones (a pattern already known for Au. africanus, the likely ancestor of sediba). I'm betting on Lucy-like interlimb proportions.

The suggestion that bipedalism is primitive isn't new or very compelling. Pickford and Senut have been trying to sell this idea via Orrorin for ages now. And Ardipithecus is an arboreal quadruped by the describers' own admission; the bipedal signal is weak to absent in my opinion.

Holly Dunsworth said...

So much for play. Now I feel compelled to comment just to cover my ath...

1. Looking forward to reading the Williams paper.

2. Still not willing to conjecture about overall arm length the way that the drawings have.

3. Of course it's an old idea. I'm not necessarily supporting it (I was playing), but I sure wouldn't throw out an idea just because it's old or not in vogue. My "ask Ardi" had nothing to do with Ardi's morphology and everything to do with the interpretations made by the authors about Miocene ape evolution in general.

occamseraser said...

Just trying to add to the discussion. No offense intended, just thought some facts and related speculations might be interesting and perhaps useful in a "comments" section.

I suspect the KW debate will continue for a while, and that's a good thing. KWing is such a peculiar way of moving quadrupedally; it's hard to imagine the degree of parallelism and number of homoplastic details required for its supposed independent development in African apes. That darn os centrale ...

Yep, hard to judge overall arm length without a standard, like relative to lower limb length or even trunk length. But the high brachial index is pretty interesting in its own right and confirms prior inferences based on fragmentary data that australopiths were not like humans in this regard. The drawings are presumably just the authors' hypothesis. It's funny, however, that Berger and Co. have gone on record as claiming that sediba had "long legs" too. The femoral fragments certainly don't confirm this, and one might wonder if it's just another effort to make sediba more Homo-like (after all, it's not important unless it's the sister group of humans, right?? <---- feeble attempt at irony).

I'd love to hear more about your take on the Miocene apes and Ardi.

Holly Dunsworth said...

Awesome. And about the last part... me too, actually. Ha!

James Goetz said...

May I add my two cents about paleontological taxonomy? Cladists must fight against the classification of another Australopithecus species. Moreover, cladists must assign a new genus to all but one Australopithecus species or some people might get confused and think that a species could be ancestral to another species, or that a genus could be ancestral to another genus, and other such madness.:)

Ken Weiss said...

Your tuppence are always welcome, Jim.
I'm not a cladist, and good ol' Holly will have to respond to that part of your comment, if she has anything to say.

I'd add that even without splitting into divergent lineages, the idea is that over time even within a lineage there is enough change that, were the ancestral and descendant individuals to be alive at the same time -- a counter-factual thought experiment, I know -- they would be reproductively incompatible and would be called separate species.

Give it enough time and they would be different enough to be said to be in different genera, though I agree that's an arbitrary, human-imposed distinction.

occamseraser said...

Precisely, and that's why Tim White has opined that sediba is "just" a chronospecies of africanus. The quest to find the magic synapomorphy that links sediba or any other australopith uneqivocally to Homo (however you wish to define the earliest version) strikes me as a tad quixotic. Playing the Homo hand is always good for headlines, however.

James Goetz said...

Yes, Ken, you're correct about hypothetical time travel and the biological species concept.:) I misrepresented cladistics, but in paleontology, cladists never assign more than one species to any given genus. And they say that we can never know for sure if any given fossil represents a specimen from an ancestral lineage of any given species. For example, paleontologists can estimate that a given fossil belonged to either A) an ancestral population or B) a closely related ancestral population. But they never know for sure that it's A or B.

James Goetz said...

Oops, I meant to say, "in paleontology, cladists never assign more than one species to any given [extinct] genus." There's no problem with classifying more than one paleontological species in an extant genus such as Homo.