William Kimbel: Paleoanthropology and Comparative Anatomy


(mouse clicks)
(soft beeping) (soft music) – [Narrator] We are the paradoxical ape, bipedal, naked, large-brained, long the master of fire,
tools, and language, but still trying to understand ourselves, aware that death is inevitable,
yet filled with optimism. We grow up slowly. We hand down knowledge. We empathize and deceive. We shape the future from our shared understanding of the past. Carta brings together experts
from diverse disciplines to exchange insights on who
we are and how we got here, an exploration made possible by the generosity of humans like you. (upbeat music) (upbeat music) – Our next speaker, Bill
Kimbel, couldn’t make it, but he sent a video, and so I’ll ask Kent
to show you the video. He’ll keep to his time more
easily this way. (laughs) (audience laughs) – [William] Good afternoon everyone. Sorry I can’t be with you this weekend, but I’m pleased to be able to talk to you about paleoanthropology,
specifically what we know, what we think we know, and what we’ve yet to fully
understand about human evolution from the point of view
of the fossil record. Obviously, given the time constraint, I can’t be comprehensive, but I’ll try to be general
rather than highly specific. I’ll identify a few
obstacles along the way and end with a short
prescription for overcoming them. Perhaps the one thing we’re
most sure of about human origins doesn’t actually come
from the fossil record, and that is that chimpanzees and humans are more closely
related to one another than either is to gorillas
and other primates. This insight, of course, comes
from the molecular evidence but is a central backdrop
to any interpretation of the hominin fossil record. We’re fairly sure that
the human-chimp divergence occurred in the late Miocene, but that date has moved
back some due to recent work on the slow down of the molecular clock within hominoids and genome-wide analysis of variation and substitution rates. These dates are consistent with the known fossil
record of early hominins. But nothing in the
molecular record can give us the chronical of events, what happened, when, where, and why, along the path from the last common
ancestor to modern humans. For this, we need strategically
programmed fieldwork and a well-sampled, precisely
calibrated fossil record. Here, I’ve added to the statement of relationships two fossil taxa, Ardipithecus and Australopithecus, that contribute critical
parts of the chronicle. Now, I’m pretty confident
in the basal position of Ardipithecus relative to
Australopithecus and Homo, though I recognize some hesitation on this point by some in our field. And the position of Australopithecus as a close relative of humans hasn’t been seriously
debated since the mid-1950s. Currently, Ardipithecus tells
us that a trio of characters, part-time bipedality, a
broad, short cranial base with a centrally located frame and magnum, and small non-honing canine teeth are foundational specializations
of the hominin clade. From the Ardipithecus perspective, Australopithecus appears to have undergone a radical makeover with
a number of features, such as lithic technology, a
significantly enlarged brain, and an expanded dietary niche that we’ve historically
associated with the Homo lineage. In my judgment, though,
the adaptive transition from Australopithecus to
Homo is modest by comparison to that between Ardipithecus
and Australopithecus. Now, it’s become fashionable to attribute all of
these changes and more, speciations, extinctions, net changes in taxonomic diversity, the appearance of biological as well as technological innovations, to high-amplitude variation
in global climatic cycles during the Pliocene and Pleistocene within a strong trend towards
cooler, drier conditions as read from paleoclimate
proxies in deep sea cores. But early hominins weren’t
adapting to climate change. They responded to changes in
local environmental conditions in the interior continents, where habitats were variably buffered against external influences by tectonic and other local to regional scale forces to a degree we’re just
beginning to comprehend. The problem is basic. We do not have a full understanding of how large-scale climate
change mediated shifts and patterns of early hominin diversity, adaptation, and geographic distribution on regional and local
scales across deep time. And we need to have this understanding if we’re to make progress in answering any of these key questions. I think the obstacles
to solving this problem lie in three broad areas,
problems of diversity, problems of sampling,
and problems of scale. None of these is a
particularly new problem, but they are persistent. One of the pivotal
climatic episodes occurred around 2.6 million years ago, with an increase in
high-amplitude climatic shifts within a global cooling trend. A popular interpretation of
the hominin fossil record suggest that there was a notable increase in species diversity as
a response to the trend, and indeed, according to a high species-diversity
scenario shown here, there were perhaps eight
species that originated within the half million year interval following the onset of the cooling trend. However, note that the time period just prior to 2.5 million years
is very sparsely populated by hominins compared to
the periods on either side. There is really only
one well-sampled species from this period, A.
africanus of southern Africa. East Africa has been recalcitrant despite a lot of teams looking. And so we really don’t know
if the sudden diversity after 2.5 million is real
or is merely an artifact of the gap in the record. But another challenge is
that paleoanthropologists are not in broad agreement
on the number of species that existed at particular
periods of time. Take for example the 3.5 to
three million year period on the other side of the gap. The high diversity scenario shows five to six separate species in this period. Some paleoanthropologists,
though, see not more than two on the evaluation of
the very same evidence. Now, it’s been suggested that this debate is analogous to asking
how many angels can dance on the head of a pin? Who cares how many species
existed in the fossil record? But to the contrary, if the diversity before
2.5 million was similar to that after 2.5 million, then it would be difficult to argue that climate change was
especially important in generating increased diversity on the heels of a global cooling event. So figuring out ancient
diversity does matter to the kinds of questions we ask. Part of the problem is
that not everyone uses the same approach or methods in their evaluation of the fossils. I’m a big fan of George
Gaylord Simpson’s scheme for inferring the existence
of fossil species. We start with an assemblage
of collected fossils. Our sample, which I must add, especially for the older
part of the record, is almost always time-averaged
on the scale of hundreds to tens of thousands of years
even in well-dated contexts. So expecting our sample to
reflect a single population on a single landscape or
ecological circumstance is usually not reasonable. But from the sample, we
need next to infer something about the parameters of the population from which the sample was drawn. Here, we need to take in
to account the possibility of change over time and
geographic variation among other factors. This is a critical step. It distinguishes a truly
evolutionary approach from pre-evolutionary typology, but it’s a step that
is too often neglected. And then, depending on which
species concept we adopt, biological, evolutionary, or phylogenetic, we might make the
inference that our sample represents a population deserving to become a formally named twig on the the tree of life, a species. Now, this is a deliberate
simplification, to be sure, but the point I want to make is that too often the intermediate step, the most difficult one for
paleoanthropology, is skipped, leading to the naming of species based on very minor
morphological deviations from other previously recognized species. I estimate that if Simpson’s
formula were rigorously applied to the hominin fossil record, perhaps 20 to 25% of the
currently named species would loose their identities as independent twigs on the tree of life, and some of my colleagues would argue that I’m still not being
conservative enough. Now, I want to say a few words
about another major obstacle, one that does not have to do with our handling of the evidence per say, but instead concerns the disjunction between the scale of the questions we ask and the scale of the evidence we use to attempt to answer them. And here I want to
acknowledge Kay Behrensmeyer of the National Museum of Natural History, whose important work on this subject deserves much better treatment than what I can offer
in the next few minutes. Here are plotted the
spatial temporal realms of some of the major questions we ask about the hominin fossil record, ranging from the truly global scale on the order of hundreds of thousands to millions of years of time, to similarly large spatial scales, down to what we might call local or ecological scale questions concerning population responses
to environmental change. Many of the phenomena of
human evolution we wish to explain are located
on the ecological scale, speciation, population dispersals, origins of adaptations, etcetera. Accordingly, it would be extremely helpful if at least some of the
evidence from the fossil record aligned with those questions. Unfortunately, for the bulk of the record, that is not the case. On the ecological scale, in which many of our most
interesting questions reside, we’re usually short-changed
when it comes to the data. We’re squeezed between on the one hand the spatially narrow window afforded by individual localities monitored over relatively short time
spans, snapshots really, and on the other hand, funnel assemblages that are almost always temporally
mixed across landscapes, aggregating individuals from different biological populations
and ecological conditions. Well, this sure sounds like a gloomy prospect
for paleoanthropology, but there are exceptional cases, like my colleague, Curtis
Marean’s excavations in South Africa, where one
of his cave sites preserves a continuous, exquisitely detailed, virtually landscaped scale paleoclimatic and archeological record
of Middle Stone Age humans in a costal marine setting between around 90 and 50,000 years ago. The sequence includes traces of the mammoth Toba volcanic eruption and presents a rare opportunity to study how early humans responded to an environmental crises
and ecological time. But this is, as I say,
an unusual circumstance. So where should we go from here? I offer a short list of agenda items that by no means exhaust
the list of important tasks for the next generation
of paleoanthropologists. It’s trite to say, but we need
more evidence from the past. But we need to be strategic in where we invest
precious funding resources to maximize the value of new information from fresh paleontological
and archeological discoveries. I have my own favorite gap
in the African fossil record between three and two million years ago, but there are others equally
worthy of time and resources, like the entire Pleistocene
of Central Asia, for example. I’m going to sound awfully
old-fashioned here, but students in human origin science need more rigorous training
in the basic principles of zoological systematics. Systematics is the bridge between the field and the laboratory, and I think paleoanthropology is a branch that the natural sciences
will suffer by ignoring it. A major step forward would
be greater cooperation and collaboration between
field research teams on questions of regional
or even continental scope. The tradition of siloed
field projects working in complete isolation from one another, even when separated by
only a few kilometers, makes no sense given the
need for broadly integrated, scaled-up approaches to the
key questions about our past. And related to this last point, we need fresh approaches to
field and analytical research that explicitly address the disjunction between the scales of
our research questions and the evidence we
extract from the earth. To finish, I’ll cite one recent
example of this approach. The Hominin Sites and
Paleolakes Drilling Project, established in the late 2000s, consists of more than 100
scientists from 11 countries who are working together
to drill, analyze, and interpret long cores from east African Rift Valley
paleolake beds proximate to early hominin sites. The goal of this work is to create a continuous high-resolution
paleoclimate link between the global scale
data from deep sea cores and the record of hominin
evolution retrieved from local sedimentary
sequences in the Rift Valley. This is important work and a model for how large integrative projects can move paleoanthropology forward. With that, I thank you for your attention. Again, I’m sorry I couldn’t be with you, but I hope this gives
you some food for thought as Carta charts its next 10 years. (audience claps) (upbeat music)

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Reader Comments

  1. Manley Nelson

    Some of the University of California's titles intrigue me but often when I go to watch one of their videos it's got some sort of far-left, feminist and or globalist agenda

  2. Manley Nelson

    Thank you University of California for just presenting some good science and facts instead of some quasi pseudo-scientific globalist agenda Masters anthropology or paleo Anthropologie or genetics or whatever else you guys often do

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