You've heard of Homo naledi, the strange "human ancestor" (really, a cousin) found a while back in South Africa. There were many skeletal remains in a cave, in the kind of shape you'd expect if they had crawled into the cave and died there, not much disturbed. They look enough like other members of our genus, Homo, to be called Homo, but if we assume that increase in brain size is the hallmark of our species, they seem to be an early grade.
Over the last ten years, we have come to appreciate the fact that our genus may have differentiated into multiple species that did not have a large brain after all, and Homo naledi is one of the reasons we think that. And, just as the "Hobbit" of Indonesia (flores) has recently been re-dated to be a bit older than people thought, Homo naledi is now dated to be a bit later than people may have thought.
For me, this is an "I told you so" moment. First, I understand, as do most of my colleagues (but not all), that a regular change over time in a trait in one lineage does not magically cause a parallel change in another lineage (though the co-evolution of a single trait in a similar direction along parallel lineages is certainly possible.) So, there was no reason to require that all later period hominins be like all other later period hominins in those later-emerging traits. Also, since no one has ever adequately explained what the heck our big brains are for, I don't subscribe to the presumption that all evolution will always evolve the big brain just because our own big brains insist that they are really cool. So, a late small brained hominin in our genus but existing long after the split with us is actually somewhat expected.
Then, there is my sense of age based on the things I've seen in the area's caves.
Some time ago, Lee Berger took me around some of the cave he had poking around in (long before this hominin was discovered) and showed me several animals that had crawled into the caves, probably looking for water during an arid period (this is already a fairly dry area). They had died in place and become mummified. In other caves, I've seen similar things, like a troop of baboons that somehow got into a cave with no known entrance and died, as well as bats that died in situ and mummified against the rock they died on.
On another occasion, Ron Clarke, another anthropologist working in the area, showed me the famous "Little Foot" which is a fossil that represents that mummy-to-stone transition, while mostly sitting on the surface of the floor(ish) of a very deep and inaccessible cave. Meanwhile, I'd been working with my friend and colleague Francis Thackeray, and he demonstrated to me how many of the diverse bits and pieces we find of australopithecines are actually probably part of individual skeletons, but discovered and excavated at very different times. These are creatures that got in the cave somehow, and were only somewhat disarticulated after death.
The whole "crawled into the cave" mode of entering the fossil record, and its presumed variant, "fell to one's death in the cave" is different from the previously presumed process of "leopard kills you, drags you onto a tree branch hanging over a cave entrance and your bones fall into the cave" means of becoming a fossil. It is of course possible, even likely, that both of these processes occurred at various times and places.
Homo naledi, according to Lee Berger, may represent a third way of getting into one of these famous caves. He suggests that the hominins themselves dragged the dead bodies of each other into the caves, as a form of treatment of the dead. That is a spectacularly controversial claim, of course, since with a small brain how can you have a god, and without a god, how can you have ritual or burial? Of course, elephants treat their dead specially sometimes, and their brain is right where it is supposed to be on the famous mouse-to-elephant curve of brain size. And, I'd bet a dozen donuts that even though Homo naledi has a small brain compared to, say, yours or mine, it is probably a good measure above that comparative curve. It was a primate, after all.
But I digress in several directions, lets get to the point. The site of Rising Star Cave, South Africa, where Homo naledi was discovered, is now dated. These things are always subject to revision and updating, but for now, it seems like we have a pretty good estimate of the age of this incredible site.
The site dates to some time between about 414,000 years ago and 236,000 years ago. That means that the site overlaps with the approximate age of the earliest, probably, modern humans. Here are the details from the abstract of the paper, published this morning:
New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of 222Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/–70 ka, whilst a minimum age scenario yields an average age of 200 +70/–61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology.
In addition to this date, it is reported that there are more fossil remains, from another cave called Lesedi Chamber. Here is the paper for that, which reports "... Further exploration led to the discovery of hominin material, now comprising 131 hominin specimens, within a second chamber, the Lesedi Chamber. The Lesedi Chamber is far separated from the Dinaledi Chamber within the Rising Star cave system, and represents a second depositional context for hominin remains. In each of three collection areas within the Lesedi Chamber, diagnostic skeletal material allows a clear attribution to H. naledi. Both adult and immature material is present. The hominin remains represent at least three individuals based upon duplication of elements, but more individuals are likely present based upon the spatial context. The most significant specimen is the near-complete cranium of a large individual, designated LES1, with an endocranial volume of approximately 610 ml and associated postcranial remains. The Lesedi Chamber skeletal sample extends our knowledge of the morphology and variation of H. naledi, and evidence of H. naledi from both recovery localities shows a consistent pattern of differentiation from other hominin species."
Since both articles are OpenAccess, you can see them for yourself. Kudos to the authors for publishing in an OpenAccess journal.
And now, back to my original digression. One gets a sense of how landscapes and land forms develop, and while this can be misleading, it is not entirely absurd to postulate rough comparative ages for things you can see based on other things you've seen. I had assumed from the way they were described originally that the Rising Star hominins would not be millions of years old. Even though Bigfoot (found by Clarke) was millions of years old and essentially on the surface (of a deeply buried unfilled chamber) I guessed that over a million-year time scale, the Rising Star material would either become diagenetically inviable as fossils or buried in sediment, or both. But over hundreds of thousands of years? That was plausible to me. In fact, I figured the remains to possibly have been even younger, and if a date half the age as suggested was calculated, I would not have been surprised.
The evolution of our thinking about human evolution went through a period when we threw out all of our old conceptions about a gradual ape to human process, replacing that with a linear evolutionary pattern with things happening in what was then a surprising order, with many human traits emerging one at a time long before brains got big. There was some diversity observed then, but the next phase of our thinking involved understanding a dramatic diverstiy of pre Homo (the genus) life forms followed by the essential erasure of variation with the rise of Homo erectus and the like. Over the last decade and a half, we are now realizing that while the later members of our genus probably did cause, or at least, were associated with, a general decrease in that early diversity, later diversity arose anyway, and there were more different kinds of hominids, very different in some cases, late into our history. Word on the street is that we can expect to learn about even more diversity in coming years.
Paul HGM Dirks, Eric M Roberts, Hannah Hilbert-Wolf, Jan D Kramers, John Hawks, Anthony Dosseto, Mathieu Duval, Marina Elliott, Mary Evans, Rainer Grün, John Hellstrom, Andy IR Herries, Renaud Joannes-Boyau, Tebogo V Makhubela, Christa J Placzek, Jessie Robbins, Carl Spandler, Jelle Wiersma, Jon Woodhead, Lee R Berger. 2017. The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa. May 2017. eLife.
Given that human DNA can be extracted from cave soil, I hope they preserve every speck of cave soil that they can so it can be sifted through to extract what ever little snips of DNA can be obtained.
This is a unique resource that will be irreversibly destroyed if it is not handled properly. Having actual DNA sequences of these ancient relatives is crucial to understanding how they fit with the rest of hominin.
We'll see. I'm sure they are digging it right, keeping samples. But this was an exposed horizon with air flowing over the surface, so there will be all sorts of DNA in any sample. Generally, conditions for this particular cave are not good for ancient DNA.
We've tried to get DNA from other older material from a nearby cave, no much luck as far as I know. On the other hand, the person I was working with doing this turned out to be not impressive, maybe others have had better luck.
Since these hominins are relatively late, maybe there is hope there.
This may not be at all relevant but I read an article recently about bird intelligence and in particular that the number of neurons and connections in a bird's small brain may equal the or exceed the numbers in much larger mammal brains.
I've also read that individual humans' brain sizes may vary by 2X. Perhaps, when dealing with different lineages of hominins, brain size comparisons -- especially from small numbers of individuals -- may not be as significant as previously thought to be.
I do not propose that this is so, I'm just wondering if it is a reasonable possibility. I can probably find the references if anyone is interested (but probably not as fast as Google).
This was the article I was thinking about. It is from caves that are younger, colder and it was mitochondrial DNA, but if entire bodies were left there, there might be some.
If you could sift through tons of soil and get (via very advanced data processing) intact genomes, that would be worth doing, even if it cost a few billion.
"Kudos to the authors for publishing in an OpenAccess journal."
Yes indeed. Well said.
David, there may be SA caves where one could do that, but not this one, there is almost no sediment.
What is needed is a way to extract DNA from limestone/flowstone like what forms inside a cave (stalactites/stalagmites). That is the best ambient DNA as it would be found in layers possibly datable independently and fairly secure. This may be impossible, however, because of the chemical environment, but if there was something preserved IN the flowstone that would itself preserve the DNA that could possibly work.
DNA might be preserved in flowstone, but you would need to extract it "carefully" and no one knows what "carefully" means.
Datable layers would be nice.
What we need is techniques that can take materials apart and sequence every single strand of DNA, and also look for DNA-shaped holes formed in the calcium carbonate and other things from molecules like DNA.
Something like an atomic force microscope that took the rocks apart molecule-by-molecule, while keeping track of everything. That would take a lot of money to develop and would take a very long time to implement.
If there is no sediment in these caves, where did it go?
If any water flowed down into these caves, it would carry sediment. That sediment is still there, or it has been incorporated into stone.
Any DNA would be associated with tissues, which would have carbonic anhydrase, which would catalyze the formation of carbonate mineralization until that mineralization blocked the diffusion of CO2 and carbonic acid. Flowstone might be an ideal site for DNA preservation. Maybe even better than sediments.
There is very little sediment in the caves. It didn't go anywhere, it just hasn't arrive there yet.
These caves are carved out by underground streams. then the stream drops down, and the cave is left as a cavity for in some cases millions of years.
Over time the occasional huge rock falls in, but that is a 10-100 million year process.
Shorter term, a cave may develop an opening near the top which allows sediment to fall or flow in. If that happens long enough, you get a decent amount of sediment. That has not happened yet in Rising Star Cave.
Eventually that sediment influx may slow as the surrounding landscape changes through erosion and the cave opening is now in a high spot instead of a low spot (this is a 500K to million year process on this landscape). Then another opening may occur, in some cases.