Have you met Naledi? Part One

I’ve been working on this post, with my best and brilliant friend, Marian Hamilton, for months now. We keep waiting for the Naledi news calm down enough that we can start to summarize what's been going on, but every time we’re ready to pull the trigger, something else hits the newsstands. Right now its shenanigans about whether the specimens could have been murdered, sacrificed, or some other form of cultural violence led to their deaths (more on that in subsequent posts).
 
Anyway, we've decided that this piece is better suited as pieces and so we split things up a bit. Part One of Have you met Naledi?  is called Naledi's 1,500 Parts. This piece summarizes news reports, science blogs, and published manuscripts describing the Dinaledi site and physical specimens that comprise Homo naledi. We will discuss the descriptive observations and measurements that we have so far and try to bring all of the pieces together (heavily referenced and linked to original sources, of course) with a little more emphasis on the science than most pop-science and media outlets and a little less intense detail than the original manuscripts from the team. The point of this section is to help situate the evidence in the context of our current understanding of human evolution.
 
Part Two (or possibly parts Two+) will be Naledi in the Crux of Controversy and will focus on a number of controversies that swirl around H. naledi. First, we’ll discuss the more controversial ideas from Berger’s team and others about how these specimens made it to the cave where they were discovered. Second, we’ll delve into the bigger debates about how we, evolutionary and paleoanthropologists in particular, do our science and report our findings. Third, we’ll talk about Berger’s hiring decisions, international reactions to a largely female excavation team, and whether this represents a case study of sexism in science.

image credit: thesecondcity.com

The location of the Rising Star Cave System and illustration of the cave itself. image credit: National Geographic

Part One: Naledi’s 1,500 Parts
 
Keeping up with current research and new discoveries can be hard, but sometimes we get those publication gems that make it easy- piercing through to the front page and attracting hoards of media attention. Even those of you who do not obsessively follow new findings in paleoanthropology have probably heard of Homo naledi, the newest member of the genus Homo and our extended ancestral family. At the very least, I bet you caught the initial excited press releases and immediately following sharp edges of media controversy surrounding this assemblage (more on that in subsequent posts).
 
With all this buzz, and the smattering of drama, it can be hard to cut through everything and keep all the information straight. The purpose of this post is to gather scholarly bits and pop-sci pieces from around the web and distill them into some solid science knowledge, Buzzfeed style:
 
8 (sets of) things to know about Homo naledi:

1. H. naledi comes from South Africa.
The taxonomic name, Homo naledi, derives from the Dinaledi chamber, or “Chamber of Stars,” where the specimens were discovered. The chamber is part of the Rising Star Cave System near Johannesburg, South Africa and is connected to the rest of the system by a single chute called the ! The system has been explored for at least 50 years and people had even entered the Dinaledi chamber multiple times prior to the 2013 discovery of this assemblage. 

2. This is a MASSIVE set of remains!
The Dinaledi Chamber assemblage includes more than 1500 bone fragments from at least 15 individuals that range in age from infant to geriatric! This single site has unearthed more bones representing a single species than almost any other ancestral hominin site and is the largest, richest assemblage every discovered on the continent of Africa. In general, Most paleoanthropological discoveries are limited to single bone or tooth, or perhaps a few clustered together. The best known and most charismatic hominin finds - Lucy the australopithecine from Ethiopia, the Nariokatome Boy (we've included pictures of each skeleton below), the Homo erectus from Kenya - are fairly complete individuals, but were discovered in isolation from others of their species. This makes it very hard for researchers to understand how varied the species might have been in terms of size, dimensions, or cranial size. Imagine millions of years from now if the only modern human to fossilize and be discovered was the 7.5-foot tall NBA star Yao Ming. What would future paleoanthropologists conclude about humans from 2015? Of course this is a very extreme example, but it illustrates the broader point: the more individuals we find of a given species, especially all together in the same place, the better understanding we have of what ‘normal’ might have been for that species. That is what makes the Dinaledi Chamber so valuable!

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Lucy (Australopithecus afarensis, specimen AL288-1) lived 3.22 and 3.18 millions years ago. She would have been just over a meter or 3 ft tall and weighed about 30 kg or 65 lbs. Image Credit: Wikipedia

The Nariokatome boy (Homo erectus) specimen is between 1.6 and 1.5 million years old. We can tell that he died at a young age because his skeleton wasn't fully formed and his third molars (aka wisdom teeth) hadn't fully emerged. Image Credit: The Smithsonian

Two of the other most fruitful hominin excavation sites, Sima de los Huesos in Spain and the Dminisi site in Georgia (the country of Georgia, of course) can help us put this assemblage in perspective. Since 1976, Sima, the largest excavation of hominin fossils to date, has yielded 28 individuals and 6,500 bone fragments. Dmanisi, also regarded as one of the most productive hominin sites, has 5 excavated individuals. The Rising Star expedition has yielded 15 individuals over two to three years at Denaledi- already tripling the number of individuals at Dmanisi. Compared to Sima, the site has recovered over half the number of individuals and a quarter of the number of bone fragments, but in about ONE 20TH OF THE TIME. Can you see why people (including Marian and I) are so excited about all this?!
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The Dmanisi skulls. Image Credit: Marcia Ponce de León and Christoph Zollikofer, University of Zurich.

A visual comparison of the bones and bone fragments recovered at Dminisi (above), Sima de los Huesos (middle), and the Dinaledi Chamber (far right). We were not able find and image that summarized everything found at Dminisi so we picked the coolest one with all the skulls. 

Sima has produced 6,500 bone fragments and teeth over the last 40 years (not all of them are shown here). Image Credit: Naturalis Historia

Here are Naledi's parts: the bone fragments recovered by the team so far. Image Credit: John Hawks, University of Wisconsin

3. This is (probably) a new leaf, or species, on the tree of human evolution.
The experts are arguing about this because that’s what they do, but it looks like we have a new species of ancient hominin. H. naledi is represented by multiple (nearly) complete skeletons. Morphologically, they are a mash-up of hominin and australopithecine traits, which sends us some mixed messages. This is to say that they don’t fit in with any other previously discovered hominin or australopithecine. They are, “Weird as hell,” to quote paleoanthropologist Fred Grine from Stony Brook University. “How so?” you ask. Well, let us tell you…This is about to get a little nitty gritty- so hold tight!

Our current ideas about the human evolutionary tree and where Naledi might fit in. Image credit: S.V. Maderis, University of Wisconsin, Madison

4. H. naledi’s mixed morphology: some parts look more like us…

1. The legs and feet: H. naledi was definitely a biped, meaning they walked upright on two feet, just like modern humans. This isn’t too surprising, as hominins have been bipedal for probably millions of years before H. naledi. The lower limb is long relative to the rest of the body with strong attachments for muscles used for walking, as seen in other members the genus Homo and separating us from earlier australopithecines. The foot and ankle, however, are shockingly similar to modern humans. The size, proportions, joint attachments, and structure of the foot are more similar to modern humans and Neanderthals than any other known species to date. The big toe is long and in line with the rest of the toes, as seen in modern humans - helpful in balance and mobility when walking upright. There is also an arch in the foot, like ours, although probably slightly lower.
2. The hands: The thumb has strong muscle attachments to the rest of the hand, like ours does, which is an important trait for manual dexterity and manipulation. Other muscle attachments and aspects of the bone morphology are also more like modern humans than seen in australopithecines, suggesting that H. naledi was a pretty dexterous species, with the ability to manipulate objects carefully and probably even make complex tools.
3. The vertebrae: The vertebrae look like later Homo, including Neanderthals, with large canals for spinal cord nerves to pass through. This large canal is present in Homo erectus and later species in our genus, and has implications for upper body motor control and breath control that could be important for running, long-distance walking, and even speaking.
4. The teeth: Two of the most obvious trends in hominin evolution are increasing brain size and decreasing molar size through time. H. naledi has a small, australopithecine-like brain size (more on this later), but rather than having large teeth like other small brained hominins we known of, the teeth are small like later members of the genus Homo! The muscle attachments and bones associated with chewing, like the jaw and cheekbones, are small in H. naledi, as in modern humans. In fact, not only does H. naledi look like other members of the genus Homo, but they look more like a modern human than other well-known later Homo members, like H. habilis and H. erectus. Nonetheless, some of the morphological traits of the tooth crowns (the tops of the teeth) and the roots are still more australopithecine-like.

From Berger et al's slideshow summarizing the eLife paper describing H. naledi showing Naledi's hands (left) and feet (above) in comparison to modern humans. Below, a mandible (lower jaw) complete with all of its teeth!

Image Credit: Siphiwe Sibeko, Reuters

5. ....and other parts look more like australopithecines.

1. The pelvis: H. naledi has a short, flared pelvis, like Lucy and other earlier australopithecines.
2.  The ribcage:  The ribs of H. naledi look more like the pyramid shape seen in australopithecines, rather than the barrel shape seen in the modern humans. This is important for paleo-anthropologists because the shape of the ribcage is one of the only ways we can reconstruct the guts of these past hominins. Australopithecines like Lucy, with their pyramid shaped ribs, still had large guts like other great apes today. This suggests a heavy-duty and metabolically costly digestive system that would have been able to process raw plant material, like fruits and leaves. Later Homo has a more barrel-shaped and narrow ribcage, indicating a shortened digestive tract associated with a higher-quality and more easy-to-digest diet (like more meat or a higher proportion of cooked foods).
3. The arms and shoulders: H. naledi has a high scapula that sits far on the sides of the back, perhaps an adaptation to moving through the trees. The humerus does not twist, as seen in modern humans, and is very slender compared to other hominin species. These primitive traits make the arms of H. naledi look more like an australopithecine than a member of Homo.
4. The fingers: Although many aspects of the hand and wrist are more similar to modern humans than they are to other extinct hominins, the relative length and curvature of the fingers are very primitive. These long, curved fingers sync with the shoulder and arm anatomy mentioned above and suggests that even though H. naledi was clearly strongly adapted for bipedalism, there were still selective pressures retaining (or at least not actively diminishing) the length and curvature of the fingers. 

Seriously, if you haven't checked out the National Geographic coverage on this, you should. Their infographics are pretty damned great. Image credit: National Geographic

A reconstructed H. naledi skull laid over an average-sized modern human. Image credit: National Geographic

6. Ok, fine, but what about brain size?
Ah, yes, the other diagnostic marker of a “true” hominin… H. naledi had a brain size of about 560cc, which is less than half the brain volume of the average human (averaging 1350cc) and barely larger than chimpanzees (275-500cc). Gracile australopithecines, like Lucy, fall just below H. naledi between 420 and 450cc, and robust australopithecines, like A.boisei, are just about the same size, averaging around 530cc. By the time we get to Homo, on the other hand, cranial volume surpassed H. naledi. The earliest member of the genus Homo (2.1-1.5 mya), H. habilis, overlaps with H. naledi at 510-660cc1 on the lower end, but by H. erectus (also classified as H. ergaster), cranial capacity is up to 850cc and growing. However, even though the size of the brain is more like earlier australopithecines than Homo, the shape of the crania is much more similar to Homo than earlier hominins.
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​7. We don't know how old H. naledi is...yet. 
Dating cave-people is hard! While caves are awesome at preserving bones and fossil assemblages, accurately dating bones that have been recovered from a cave - particularly this cave - is prohibitively difficult. Why? Well let’s take a look our current toolkit for dating fossils:

1.  Carbon-14 (or radiocarbon dating): Probably the most well-known dating technique for archeological specimens, this method relies on the predictable decay of the radioactive isotope of carbon (carbon 14) in organic tissues after death. The major problem with carbon-14 dating is that is can only go back around 50,000 years because, due to the half-like of carbon-14, there isn’t enough left in specimens any older than that. It might seem like that would still be a good start for the Homo naledi specimens but carbon-14 dating is also a destructive dating technique, which is a major drawback. This research team has ruled out destructive sampling until they could describe the skeleton and finish 3D scanning all of the bone fragments. It is likely that the team will try it eventually, just to rule out a shockingly young date for the specimens, but as it is highly likely that the samples are older than 50,000 years, it is unlikely that it will tell us much of use.
2. Uranium/thorium dating: Again relying on our favorite process of radioactive decay, this dating technique measures uranium in calcite rocks surrounding the fossils. Over time, uranium decays into thorium and lead at a predicable rate. While the fossils themselves are in a soft and sadly un-datable gravel rock matrix, there is a calcite flowstone laying on top of the layer where the fossils are located. If researchers can date this layer, we will have a minimum date. The problem is, we don’t yet know if there is another flowstone layer under the fossil layer. If there isn’t, we won’t have any way to get a maximum date for the fossils. In other words, dating the top flowstone could tell us that the fossil cannot be any younger than, say, one million years - but we wouldn’t be able to know if that meant they were two million or ten million years old without another flowstone underneath.
3. Biochronology: This technique has great advantages because its not destructive and can be very reliable. For example, the Dmanisi site we mentioned earlier has used this technique to determine that the age of the homins they’ve found is somewhere between 1.8 and 1.7 million years old. That is an incredibly, wonderfully narrow window for paleoanthropologists. However… this technique requires the discovery of known animal skeletons alongside hominin remains and that we have a good idea about when those animals lived. Unfortunately, the only animal skeletons that have been found alongside H. naledi are a few rodents and a weird owl. Neither of which are particularly helpful for this dating technique. 
4. Biostratigraphy: A similar approach is to use reliably-dated fossil assemblages from the same rock layer as the assemblage of interest. The basic idea is that if two groups of fossils were buried under the same layers of rock and soil, then they're probably about the same age. Unfortunately, the sorts of conditions that make caves so great for preserving stuff, also lead to rocks and soil deposits forming differently and this rules out biostratigraphy. 
5. Magnetic dating: Perhaps the most promising technique, this relies on small pieces of iron in the rock matrix indicating shifts in the magnetic poles of the earth. We know very well when these shifts happened, so finding these flipped pieces of iron would allow researchers to place the rock matrix (and thus the fossils) in the timeline. However, this is a very laborious and time-intensive technique, so we shouldn’t hold our breath for strong dates anytime immediately.

Image Credit: Curtis, Eye Witness News

8. “But, wait a second, didn’t I just see a report that Naledi was 2-3 million years old?”
Yes, impressively well-informed reader, you did. A recent study by Thackeray compared a composite measure of cranial morphology between H. naledi and other human ancestors, placing H. naledi between two other hominins: H. rudolfensis and H. erectus. The maximum recorded age for H. rudolfensis is about 2.5 million years and H. erectus is considered to be about 1.5 million years old. As such, the author concluded that the H. naledi branch must be somewhere on the order of 2 million years old. However, this method of comparing the shape and size of H. naledi bones versus other, more confidently-dated specimens, is kind of sketchy. Now, this is a touchy subject among paleoanthropologists and we’re happy to discuss this in depth if anyone is interested, but several studies have shown that phylogenetic trees based on morphology do not faithfully recapitulate genetic relationships. In other words, it isn’t a great method for determining true genetic relation or the position of the branches on an evolutionary tree. Given the mosaic nature of H. naledi’s skeletal features, this type of dating may be particularly problematic. Think about it this way: when the mosaic of traits exhibited by a single set of skeletons is representative of two different genuses that span a few million years, which features can you choose as the “most” relevant? The classic approach is to look for the most "derived" traits, or the traits that are present in more recent ancestors, but not older ones. However, its pretty easy to see how this sort of classification can get real circular real fast. And that's why we’re on the side of keeping this issue open for discussion.

Image credit: Dog on the Moon for the Guardian

 
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Ok, so obviously this cartoon is ridiculous- I mean, the hominins are dogs. But this cartoonist (Dog on the Moon) drew up a pretty hilarious piece depicting all the to-do about Naledi. Check out the full cartoon, but only if you promise to remember that it's satire. 

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So to recap this hodgepodge of a hominin: Homo naledi had human-like feet and long, strongly muscled legs adapted for striding, walking, and perhaps even running, but a shoulder joint and long, curved fingers that look more like a species spending time in the trees. They had flaring, australopithecine-like hips and a wide pyramid-shaped ribcage, but Homo-like vertebrae. Their teeth were small like ours but simple in their shape and structure; their brain size was small like earlier species, but the shape of the cranium shared many traits with other members of the genus Homo.
 
The real question is: what are we to make of all this? First, the mixed morphology of Naledi flies in the face of popular notion that there was a “human adaptive package” - a suite of traits that arose all together that shifted us from not-human to human. Typically, these traits include larger brain size, hand adaptations for increased dexterity, smaller dentition and smaller guts, and adaptations for efficient long-distance walking and running. H. naledi makes it pretty clear that these traits did not arise all together, and even throw into question the logical order in which such traits would have arisen. For example, most people assume that large brain size was a prerequisite for sophisticated tool use, and then anatomical adaptations for fine motor skills and manipulation would have come afterwards. With their human-like hands but australopithecine-like brains, H. naledi looks from the past and suggests, “Maybe not…”
 
Second, no matter how old these specimens are, H. naledi is an incredible find that will revise the story of human evolution. If they preceded humans we might have identified a direct ancestor to our own species. If they lived alongside early Homo, that could have profound implications for the “bushiness” of the hominin branch of the primate tree. However, our ability to fit this evidence into the context of our evolutionary history is severely limited until we find a decent method for dating these specimens.
 
No matter what—this is some very exciting science! Stay tuned for more. 

Please note: comments and criticisms for this post are welcome, as well as suggestions for future posts. Our goal is to help evolutionary anthropology and science in general reach a wider audience and are always excited for opportunities to do a better job!! 

​Many of the sources used to write this piece are linked directly to the text. Here is a list of those sources plus some additional resources for any and all of those interested: 

Scientific articles: 
1. Berger, L. R., Hawks, J., de Ruiter, D. J., Churchill, S. E., Schmid, P., Delezene, L. K., ... & Skinner, M. M. (2015). Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. Elife, 4, e09560.
(And associated slideshow: http://www.slideshare.net/Lor342/homo-naledi-highlights-from-the-paper-published-on-elife)
2. Gibbons, A. (2015). New human species discovered. Science, 349(6253), 1149-1150.
3. Harcourt-Smith, W. E. H., Throckmorton, Z., Congdon, K. A., Zipfel, B., Deane, A. S., Drapeau, M. S. M., ... & DeSilva, J. M. (2015). The foot of Homo naledi. Nature communications, 6.
4. Kivell, T. L., Deane, A. S., Tocheri, M. W., Orr, C. M., Schmid, P., Hawks, J., ... & Churchill, S. E. (2015). The hand of Homo naledi. Nature communications, 6.
5. Stringer, C. (2015). The many mysteries of Homo naledi. eLife, 4, e10627.
6. Thackeray, J. F. (2015). Estimating the age and affinities of Homo naledi.South African Journal of Science, 111(11 & 12), 1-2.

Other sources: 
Team Leader, Lee Berger's website: http://ewn.co.za/Features/Naledi
Paleoanthropologist and team member John Hawke's Blog: http://johnhawks.net/
The Expedition Blog: http://voices.nationalgeographic.com/blog/rising-star-expedition/
National Geographic Coverage: http://news.nationalgeographic.com/2015/09/150910-human-evolution-change/
Other news releases: 
http://theconversation.com/homo-naledi-may-be-two-million-years-old-give-or-take-50416
http://www.sciencemag.org/content/349/6253/1149.full
https://fsp.trinity.duke.edu/projects/new-fossil-discovery-redefines-what-it-means-be-human
Other blogs about Naledi: 
http://ncse.com/blog/2015/10/cave-homo-naledi-textbook-example-how-to-do-science-0016693

For some words about Naledi from Berger's own mouth, check out this video!