The Ape In the Trees

I'd guess that few people outside of primate paleontology are familiar with the story of The Ape in the Tree.   It's one of those stories in paleontology that never gets old—even if you've heard Alan Walker, one of the story's main unravelers and my Ph.D. advisor, tell it many times. Now, thanks to a new discovery by our team, out today in Nature Communications, we've got a new story to tell alongside it: The Ape in the Trees. 

Alan Walker in the mid 80s excavating at the Kaswanga Primate Site, Rusinga Island, Kenya

Because it's a juicy one, let's recount the ol' Ape in the Tree first.

The "Ape" refers to Proconsul, a genus of fossil hominoids from the earliest part of the ape radiation. Proconsul remains are preserved in deposits down at the early part of the Miocene epoch at sites in Kenya and Uganda. These animals weren't doing much of any brachiation, suspension, or knuckle-walking in the trees or on the ground like extant gibbons, siamangs, orangutans, gorillas, chimpanzees and bonobos. They were what we call generalized arboreal quadrupeds, moving about the trees by relatively slow but strong grasping with their hands and feet, and without the aid of a tail for balance. Body size estimates across the few known species range from 9-90 kg (20-200 pounds), but little in the way of functional anatomical differences (which are translated into behavioral differences) are apparent.

Proconsul skull discovered by Mary Leakey on Rusinga Island in the late 1940s.

Mary's skull was commemorated as a postage stamp.

In the story, "Ape" specifically refers to just one partial skeleton found at R114 on Rusinga Island, Kenya.

KNM-RU 2036. Proconsul partial skeleton from site R114. The green bits are the additional parts found by Walker and colleagues decades after the initial discovery marked in blue.

Rusinga's an island in Lake Victoria connected by a short human-built causeway to the mainland, but the lake, and hence the ability for Western Kenyan land to become surrounded by its water, is fairly recent, within the last 2 million years or so, and it's fluctuated greatly in depth ever since. Here's how it looks now:

© Shipman and Walker

© Shipman and Walker

20-18 million years ago, at the time of Proconsul, Rusinga Island's terrain was part of the land flanking the then-active Kisingiri Volcano which is just south of Mbita and out of frame of the map above. It's in large part thanks to the volcanic deposits that such wonderful preservation occurs on Rusinga.

Two fossilized grasshoppers found preserved in a deinothere (ancient elephant relative) footprint near the R114 (“ape in the tree”) site on Rusinga Island, Kenya. The one in the back is flipped upside down. © Alan Walker

Articulated bones of the right and left feet of an adult Proconsul (museum catalog no. KPS III). These ape feet remained intact upon burial and during fossilization, and were excavated roughly 20 million years later at the Kaswanga Primate Site, Rusinga Island, Kenya. © Alan Walker

People have been studying Proconsul for nearly 90 years (with the initial find at a limestone quarry in Koru, Western Kenya). And they've been studying the fossils and rocks of Rusinga Island, the site which has produced the most Proconsul fossils, for nearly as long.

Site R114 was recognized early on in the scientific history of Rusinga Island. A few curious circular deposits of a different rock composition than the surrounding matrix (and containing interesting fossils including Proconsul KNM-RU 2036, the blue bits on the figure above) were hypothesized to be potholes. This was the explanation that held until the mid 1980s when Alan Walker and his friends started poking around in the Rusinga collections in Nairobi.  Walker noticed some Proconsul bones collected by Louis Leakey and colleagues from Rusinga Island had been misidentified as other animals. This inspired a joint team from Walker's university, the Johns Hopkins University, and the National Museums of Kenya to go to Rusinga Island to re-examine sites there. The expeditions were successful. They recovered more Proconsul bones to join the partial skeleton KNM-RU 2036 (the green bits on the figure above).  But what's even more exciting was their investigation of this pothole idea.

They simply (I meant that in spirit, not sweat) dug down at the edges of the main pothole, the one that produced the Proconsul bones, and found something surprising. Instead of finding a basin shape in the ground like a good pothole, the edges actually got wider the deeper they dug.

The pothole that wasn't. (R114, Rusinga Island)

Instead of a pothole, this curious fossil-filled feature appears to be a fossil tree trunk that was both filled in by sediment and fossils but also buried deep by surrounding sediment. To explain how the Proconsul and other creatures, like a rabbit and an ungulate, were also preserved, Walker and colleagues surmised that it was once hollowed out and used by a carnivore, like a creodont, as a den.

Figure from Alan Walker and Mark Teaford (1989) The Hunt for Proconsul. Scientific American 260(1): 82. Diagram drawn by Tom Prentiss. Also reproduced in Walker and Shipman’s 2005 book The Ape in the Tree. 

So there you have it... that's "The Ape in the Tree" in a nutshell... You can read much more about it, and other stories about Proconsul, in the book of the same name.

link

Now, onto the brand new story of The Ape in the Trees...

A team of us has been working the sites on Rusinga and nearby Mfangano Island, annually or more, since 2006. We've been fortunate to collaborate internationally and to share this work with many undergraduate and graduate students too.

Systematic survey at site R107 in 2011

We've been exceedingly lucky to have a geologist on our team, Dan Peppe, who is also a paleobotanist. This is because there are not just fossil tree trunks but exquisitely preserved leaves.

Figure from "A morphotype catalog and paleoenvironmental interpretations of early Miocene fossil leaves from the Hiwegi Formation, Rusinga Island, Lake Victoria, Kenya" [link]

And—as discovered at R3, a particularly rich site, by Peppe's doctoral student, Lauren Michel—there are also well-preserved root systems. And these are located among fossil tree trunks, within nicely preserved paleosols (soils) that happen to contain fossils of Proconsul and another primate Dendropithecus.

Lauren Michel and Dan Peppe hard at work.

That's what we've published today in Nature Communications.

Reconstruction of site R3, this fossil forest containing fossil Proconsul (fore) and Dendropithecus (higher up) by artist Jason Brougham.

This time, then, the story's not about an ape skeleton actually inside a tree, but it's about an ape (and other creatures) dying in a preserved fossil forest of trees. The root systems and distance between trunks points to a closed-canopy forest, meaning that the arboreal creatures like Proconsul and Dendropithecus needn't come down to the ground to travel and that the ground was most likely very shady, with an ecosystem that reflected it. Further, the fossil leaf morphologies point to a wet and warm forest. All consistent with what many primates, especially apes, prefer for habitat today. What's more, this habitat reconstruction jibes so nicely with the behavioral interpretations we've made for Proconsul based on anatomy.

We started this long-term project at such historically well-known sites not just to find more of the ancient fossil apes they're famous for (which we have!), but because we wanted to describe the paleoenvironments in which they lived, died, and evolved. The findings in this paper are more than I dared to dream would come from our work already. It's some of the best evidence linking ape to habitat that we could ask for. It really speaks to the power of collaboration and perseverance.

Glorious fossils. Not too shabby of a campsite either.

*** 

To get a sense of what we do at Rusinga, here's a nice film that a crew from the American Museum of Natural History put together about our work.

Science Bulletins: Expedition Rusinga - Uncovering Our Adaptive Origins from AMNH on Vimeo.

Our work's also featured in the Hall of Human Origins at the AMNH, including a fun interactive exhibit that shows us doing many fieldwork type things like sieving endless piles of dirt.

And here's a peek at the ape tail loss story that Proconsul tells, which will be part of Neil Shubin's three-part series in April on PBS called Your Inner Fish.

***

The paper

Michel, L. et al (Feb 18, 2014) Remnants of an ancient forest provide ecological context for Early Miocene fossil apes. Nature Communications doi:10.1038/ncomms4236

Paleontology in action

I posted some photos from my fieldwork over on Facebook because I can't get them to upload to Blogger from my field internet connection. You don't need an account to see them. Enjoy!

Researchers call B.S. on Ardi’s habitat

Raise your hand if you think I’m talking about this Ardi.

Well, I’m sure that there are plenty of people calling B.S. on his parents, but that’s not the Ardi I’m talking about.

This is the Ardi that I’m talking about.

Ever since the papers were published by White’s team in Science last fall, we’ve been waiting for the public debate to unfold and now it's well underway with two Technical Comments in Science last week.

The first paper questions the phylogenetics and functional morphology of Ardipithecus (to be discussed in a later, separate post) and the second, by Cerling, Levin et al., questions the paleoenvironmental interpretations for the creature from Aramis, Ethiopia. Both were published alongside rebuttals by White’s team.

Let’s talk about habitats first…

White’s team originally claimed that Ardipithecus habitats ranged from woodland to forest patches and that, “…early hominids did not evolve in response to open savanna or mosaic settings.”

“So what?” you might be asking.

Well, it's a big what! If Ardipithecus is indeed an early biped, then bipedalism did not evolve in a grassy place, but rather a wooded one.

Ever since the 1960s, the prevailing hypothesis, known as the “savanna hypothesis,” has more or less been used to explain the evolution of human-ness.

Savanna-living, according to the savanna hypothesis, is what separated hominins—the exclusively human branch on the evolutionary tree—from the other African apes. Moving about on the ground, rather than in the trees, was a selective force in the evolution of bipedalism. And, of course, there are all of the other evolutionary ramifications of a shift towards savanna ecology. So if Ardipithecus did not live in a savanna, then this is some major evidence refuting the savanna hypothesis. But if White’s team is wrong, then the hypothesis lives on.

Well, what’s a savanna? It’s actually much broader than “grassland” and can include wooded aspects as well. Here is how the United Nations Scientific and Cultural Organization (UNESCO) classifies and defines African vegetation (as summarized in Cerling et al.):

1. Grassland is land covered with grasses or other herbs, either without woody plants or the latter not covering more than 10 per cent of the ground. (What people call “savanna.”)

2. Wooded grassland is land covered with grasses and other herbs, with woody plants covering between 10 and 40 per cent of the ground. (Also what people call “savanna.”)

3. Scrub woodland has a canopy height less than 8 m, intermediate between woodland and bushland. As proportions of bushes, shrub, and grasses increase, woodlands grade into bushland/thickets or wooded grasslands (above). (Savanna-ish)

4. Woodlands have trees with canopy heights of 8 to 20 m; their crowns cover at least 40% of the land surface but do not overlap extensively. Woodland ground layer always includes heliophilous (sun-loving, C4) grasses, herbs/forbs, and incomplete small tree and shrub understories.

5. Closed woodlands have less continuous canopies and poorly developed grass layers.

6. Forests have continuous stands of trees with overlapping crowns, forming a closed, often multistory canopy 10 to 50 m high; the sparse ground layer usually lacks grasses.

Okay, first of all, both teams (both Cerling et al. and White et al.) agree that open savanna grassland was not the environmental context of Ardipithecus. In other words, number 1 is out.

However, White’s team says that the conditions at Aramis were between numbers 4-5, but Cerling and Levin’s group prefer 2-3 which include a patchy riparian forests set within a dry, savanna landscape.

[What’s at stake here is the issue of interconnectivity: Could a hominin move from tree to tree through the forest canopy without coming down to the ground, or not? In choices 5 and 6 above, it could, but in 1-4, it couldn’t.]

Researchers on both sides of this debate address the gamut of information used to piece together the scene...

Some Major Sources of Paleoenvironmental Evidence

1. Stable carbon isotopes in paleosols (ancient soils)
2. Oxygen isotopes of mammalian tooth enamel used to determine paleoaridity
3. Carbon isotopes in mammalian tooth enamel
4. Relative abundance of phytoliths (microscopic silica particles from plants)
5. Relative abundance of micromammal fossils
6. Vertebrate species: types of birds, large and small mammals, gastropods and fossil wood preserved in association with Ardipithecus.
7. Vertebrate morphology: For example, Browsers (tree and shrub eaters) vs. Grazers (grass eaters)
8. Geology and taphonomy of fossil assemblage (How the fossils got there)

However, Cerling and Levin’s team emphasize the importance of 1-4, while White’s team emphasizes 5-8.

Regarding 1-4, there is a fundamental disagreement between the two camps over what samples, both ancient and modern, to include in the analyses. Both have used the same data, but with different comparison data or on different scales (regional vs. small-scale habitat) and have come to very different conclusions.

White et al. end their rebuttal with a comment on the implications of their results on the savanna hypothesis.

“The vision of apes trekking bipedally between increasingly isolated forest patches has maintained its allure across decades of research. The repeated discovery of obligatorily bipedal, megadont Australopithecus in later open habitats erroneously reinforced this notion of bipedality’s beginnings. It was perhaps inevitable that proxy records reflecting global shifts in carbon isotope values would be postulated as the missing piece of the puzzle of hominid origins (22).”

That sounds a little like a dig at Cerling’s team’s work, citing their 1997 Nature paper. But I could just be overly sensitive to these things. Anyway, White et al., continues…

“By focusing too coarsely on the regional environment, Cerling et al. seem to overlook evidence that differentially and consistently links Ardipithecus to a woodland habitat and thereby distinguishes it ecologically from Australopithecus. We contend that compared with Ar. ramidus, Australopithecus was more ecologically flexible, probably ranged more frequently and further into the open environments that Cerling et al. term “tree or bush savanna,” and evolved remarkably distinct and highly derived dietary and locomotor adaptations to this end. After assessing the totality of the pertinent environmental and ecological evidence, we concluded that Ar. ramidus preferred the more wooded habitats among the available spectrum in the regional geography: “...the integration of available physical and biological evidence establishes Ar. ramidus as a denizen of the closed habitats along this continuum”. The Aramis evidence is not easily accommodated by an environmentally deterministic view that involves globally shrinking forests spawning savanna-striding hominids. We contend that this narrative is now undermined by the totality of data from 4.4-Ma Aramis. These rich, diverse data are spatially and chronologically intimately associated with Ardipithecus, thereby providing an unparalleled view of the early hominid niche within the larger geographic setting.”

Until others can grasp the “totality of data” from 4.4-Ma Aramis in a similar way, I suppose this new view of early hominin evolution will remain something to “contend” rather than something for others to readily see and accept.

References

Thure E. Cerling, et al. Science 328, 1105-d (2010); Comment on the Paleoenvironment of Ardipithecus ramidus.

Tim D. White, et al. Science 328, 1105-e (2010); Response to Comment on the Paleoenvironment of Ardipithecus ramidus.

You look just like your grandma, Or, Why conservation poses challenges for evolutionary biology

We posted last week about the 95 million year preservation in amber of plants and insects in recently discovered samples of ancient amber in Africa. This showed the early distribution of recognized types of organism very long ago in Africa, affecting reconstructions of paleoecology, since at least some had not formerly been known to be there.

That was interesting, and we raised another broader point that these specimens showed. It has to do with the conservation of form over evolutionary time. It's one thing to look like your granny, and quite another to look like your 95-million-times-great granny!

The evolutionary problem (and it's not a new one nor one we invented) is to explain the very ancient existence of species that look strikingly, perhaps entirely, those alive today. If (as we recently discussed) even a few hundred thousand years is enough for anthropologists to insist on naming a fossil as a new species--or even genus!--then how can something 95My old look so strikingly like what we see today?

One might think that insects, because they have a smaller genome than vertebrates, have less genetic 'room' for variation, and are more constrained by selection so that organisms like flies have no variation left--no ability to viably change form. Bacteria have maintained their form since literally the most ancient of times (fossils called stromatolites, ~3Bya old). But even bacteria have multiple genes contributing to traits, and insects and plants certainly to, too. Selection studies in these types of organisms, and mapping studies on even such exotic traits as startle reflexes, wing venation, bristle count, and sleep habits in flies show genetic causal complexity due to existing variation in their natural populations.

Thus, with many contributing genes, each susceptible to variation, one might expect plenty of room to vary, ever so slightly, over millions of years. Furthermore, these groups have continued to evolve diverse species so that there wasn't just one way, say, to be a fly that, once installed, was so tightly fenced in by natural selection as to be preserved for time immemorial. In genetic terms this would generally called an adaptive peak on the genetic fitness landscape, that the species simply couldn't get off. But this is a post-hoc argument that seems just too pat, without some further justification (speculation about this has certainly been offered, but is difficult to prove).

Anyway, the conservation of form and function, as well as its adaptive evolution present important problems that are not yet fully solved. You look a lot like your granny, and we have a clear understanding of why that is. But you don't look exactly like her, and for a fly or fern to look like its countlessly-great-great grannies is harder to explain.