In 1978, a paleoanthropological team including Mary Leakey, Richard Hay, and Tim White made a startling discovery at Laetoli, Tanzania; in a bed of volcanic ash that would later be dated to about 3.5 million years old were the footprints of ancient hominids. The preserved trackway, found to contain the footprints of three individuals of the same species walking in the same direction during a very short period of time (possibly walking together as a group), would become one of the most important and iconic of hominid fossils, the fact that hominids were walking upright 3.5 million years ago being unmistakably hewn into the rock. The find has not been without controversy, however, everything from the identity of the trackmakers to the world in which they lived being called into question, but today a sharper picture of ancient Laetoli is coming into view, one that challenges one of the most cherished and long-held ideas of human evolution.
If we're to understand the significance of Laetoli in terms of human evolution and scientific inquiry, we need to leave the 1978 trackway discovery for a moment and look at another, older, significant find. In 1934, German anthropologist Ludwig Kohl-Larsen discovered a hominid lower jaw at Laetoli, the fossil (LH4) being known today as the type specimen of Australopithecus afarensis (the genus to which the famous remains of "Lucy" belong). This made the later discovery of the trackways indicative of a bipedal hominid at Laetoli very surprising indeed; A. afarensis seemed to be more ape-like than human-like*, so how could it have been the creator of the footprints? While the view that has gained the most wide acceptance today is that members of the species known as A. afarensis created the tracks, others have suggested that they were made by a currently unknown species of Homo that is still awaiting discovery, but there is no evidence for such a phantom fossil at present. It is certainly a reasonable inference, then, that A. afarensis created the tracks, although detailed comparison with the feet of A. afarensis will be a more definitive test. Still, ichnofossils("trace fossils") are traditionally given their own names as we can never be 100% certain that the tracks were made by any animal that we know of today. For example, a large theropod track from Cretaceous-aged rock in New Mexico was almost certainly made by Tyrannosaurus rex but was given the name Tyrannosauripas pillmorei as no one was present to document the formation of the track despite the strong support for the association of Tyrannosaurus and the print.
*As I've written before (and as others more learned than I have stated) looking at hominid fossils in terms of being more ape-like or more human-like is a mistake that immediately brings subjective bias into research. Hominids have to be studied as creatures within their own temporal and ecological contexts, not as privileged genera that we've awarded the honor of being our ancestors. Especially when considering variation and convergence, looking at hominids only through the filter of how close to Homo sapiens they are will only cause taxonomic and evolutionary messes that will be difficult to clean up.
Even if we can't have absolute certainty about the identity of the trackmakers of Laetoli at present, the tracks themselves can tell us much about the functional morphology of the creatures that walked through the volcanic ash. While the tracks are very small, the two more easily distinguishable prints being between 18 and 22 cm long, they show some remarkable characteristics that prove that the hominids were walking upright on two legs. First, there are no impressions of knuckles on the ground, indicating that these animals were not moving in the manner of modern day Chimpanzees, Gorillas, or Bonobos. More importantly, however, the big toe is brought in line with the rest of the toes at the front of the foot and does not jut out to the side as in extant great apes. The condition of the toe is not as derived as in humans or later bipedal hominids, but the difference between the Laetoli foot structure and the foot structure of living apes is remarkable. The footprints are not simply flat impressions, either; they can tell us a bit about how these animals walked. They did not shuffle or stop, but rather walked by placing the heel down and then pushing off with the front of the foot and toes (hence the move of the big toe), again proving that the tracks were made by creatures walking on two feet. Indeed, hominids were upright, obligate bipeds by 3.5 million years ago at the latest, but what sort of world did the "upright apes" inhabit?
One of the most popular notions of human evolution is the "Savanna Hypothesis," an idea that looms large in our consideration of bipedalism. Created by the discoverer of the famous Taung Skull, Raymond Dart, the gist of the savanna hypothesis is that for one reason or another (be it shrinking forests due to ecological change or more opportunities in the grasslands) our ape ancestors left the relative safety of the forest for the harsher African savannas. Faced with a hotter, harsher climate and the constant presence of large predators, our ancestors began standing up to carry tools or spot predators, bipedalism evolving as a result of the need to stand up while moving across open spaces. Such an idea seems to make sense and invokes a somewhat appealing evolutionary narrative, and it is a very difficult idea to shake. While it is a parody of the evolutionary ancestry of our species, the recent "Simpsons Evolution" intro is a good example of the savanna hypothesis and "March of Progress" mythology rolled into one*;
*It should be noted, however, that the Simpsons intro seems to have been derived from (or at least convergent with) the introduction to a 1991 Richard Dawkins lecture entitled "Waking Up in the Universe".
Recent studies of the paleoecology of Laetoli, however, reflect a much different picture of the habitat that A. afarensis inhabited. While earlier studies seemed to support the idea that Laetoli was a grassland 3.5 million years ago not much different from the same area today, a newer picture of a mosaic of habitats has emerged, one that harbored a wide diversity of taxa. While it is not complete, here is at least a partial list of taxa known from the Upper Laetolil beds (the beds in which the footprints were found) from Laetoli;
- Carnivora; Hyenas, Lions, and the "false sabercat" Dinofelis
- Primates; Baboons, Australopithecus afarensis
- Artiodactyls; Suids (Notochoerus euilus), Giraffe (Giraffa jumae, Giraffa stille, Sivatherium maurusium), Gazelle, Antelope, & Buffalo (Tragelaphus sp., Simatherium kohllarseni, Bravobus nanincisus, Cephalophini sp., Praedamalis deturi, ?Hippotragini, Parmularius pandatus, Alcelaphini sp., Madoqua avifluminis, ?Raphicerus sp., Gazella janenschi, Pelea sp.)
- Perissodactyls; Rhino (Ceratotherium praecox), Chalicothere, Horses (Hipparion sp., Eurygnathohippus sp.)
- Proboscidea; Deinotherium sp., Anancus kenyensis, Loxodonta exopata
- Lagomorphs; Rabbit
- Aves; Ostrich (Struthio sp.), Francolins, Guineafowl
- Insects; Termites (Macrotermes sp.)
Some species or even entire groups of animals might be absent or yet to be discovered due to taphonomy, but what the fossils discovered thus far show a diverse assemblage of animals that preferred a mosaic woodland/bushland/grassland type of habitat. Reconstructing the precise diet or behavior of extinct fauna can be tricky, but in the case of Laetoli, intertwining lines of evidence seem to reflect a more wooded habitat than savanna-like one. Take the presence of Guineafowl and Francolins, for instance. Both of these types of birds belong to the Order Galliformes (pheasants and turkeys are also representative of the group) and seem to prefer a mix of open and wooded habitats. While they may forage on open ground during the day, they prefer to sleep in trees at night and are not exclusively ground birds. The presence of eggs of these birds, some fragmentary and some whole, shows that they were present at the Laetoli site and probably inhabited a niche similar to that of their modern day relatives. 3.5 million years is a long time, however, and perhaps the behaviors or habits of the ancient Galliform birds were different from their modern-day relatives, but thankfully there is even stronger evidence from the mammals present at the site for a mosaic woodland habitat.
The herbivorous mammalian fauna of the Upper Laetolil beds appears to consist of a combination of browsers and grazers, isotopic studies revealing that they fed on both C3 and C4 plants. There is a lot more to such studies than I can give attention to here, but C3 plants are typically indicative of woodland or forest habitats and C4 plants indicate grasslands, and the strong presence of C3 plants from Laetoli strengthens the idea that the area was a mosaic rather than exclusively savanna. The habitat would change over the next 1 million years, however, the Upper Ndolanya beds of Laetoli reflecting an overall change in proportion of habitats. While woodland was still present, it was much reduced from its expanse at 3.5 mya, the ecology of the Ndolanya beds more closely resembling what was originally hypothesized for Laetoli. This change raises an important question, however; if Laetoli did not become more like a savanna until about a million years after the hominid trackway was made, how significant was the savanna to the development of bipedalism?
The debate about how bipedalism initially evolved and what the hominid interaction with the savanna habitat was will continue for some time, but the paleoecology of the Upper Laetolil beds of Laetoli show that grassland habitats were probably not as significant to the development of bipedalism among hominids as suggested by Dart and others. If A. afarensis was indeed the trackmaker, then it seems that the transition to a bipedal gait began in the forest or woodland before 3.5 mya rather than a direct reaction to the shrinking woodland reflected by the younger Ndolanya beds. I suspect, however, that the savanna hypothesis of the origins of bipedalism will remain with us for some time, the power of an evolutionary narrative sometimes being more powerful than the current state of science on the subject. Such problems plague the evolution of the horse and birds as well, the public understanding of evolution existing in a sort of intellectual quagmire that lags behind the latest research and discoveries. We should be careful, then, not to pen evolutionary "Just-so stories" for we can never known when a new fossil will come out of the ground and force us to undertake a rewrite.
References;
Berge, C., Penin, X., and Pellé, E. "New interpretation of Laetoli footprints using an experimental approach and Procrustes analysis: Preliminary results" Comptes Rendus Palevol. Volume 5, Issues 3-4, March-April 2006, Pages 561-569
Darlington, J.P.E.C. "Distinctive fossilised termite nests at Laetoli, Tanzania" Insect. Soc. 52 (2005) 408-409
Harrison, T. "Fossil bird eggs from the Pliocene of Laetoli, Tanzania: Their taxonomic and paleoecological relationships" Journal of African Earth Sciences. 41 (2005) 289-302
Harrison, T, and Msuya, C.P. "Fossil struthionid eggshells from Laetoli, Tanzania: Taxonomic and biostratigraphic significance" Journal of African Earth Sciences. 41 (2005) 303-315
Kaiser, T.M. "Proposed Fossil Insect Modification to Fossil Mammalian Bone from Plio-Pleistocene Hominid-Bearing Deposits of Laetoli (Northern Tanzania)" Annals of the Entomological Society of America. Volume 93, Issue 4 (July 2000) pp. 693-700
Kingston, J.D., and Harrison, T. "Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: Implications for early hominin paleoecology" Palaeogeography, Palaeoclimatology, Palaeoecology. 243 (2007) 272-306
Kovarovic, K, and Andrews, P. "Bovid postcranial ecomorphological survey of the
Laetoli paleoenvironment" Journal of Human Evolution. 52 (2007) 663e680
Kovarovic, K., Andrews, P., and Aiello, L. "The palaeoecology of the Upper Ndolanya Beds at Laetoli, Tanzania" Journal of Human Evolution (2002) 43, 395-418
Leakey, M.D. "Tracks and Tools" Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, Vol. 292, No. 1057, The Emergence of Man. (May 8, 1981), pp. 95-102
Lewis, ME, and Werdelin, L. "Patterns of change in the Plio-Pleistocene carnivorans of eastern Africa: Implications for hominin evolution." Hominin Environments in the East African Pliocene: An Assessment of the Faunal Evidence. Springer Netherlands, 2007, pp. 77-105
Marean, CW; Ehrhardt, CL "Paleoanthropological and paleoecological implications of the taphonomy of a sabertooth's den" Journal of Human Evolution. Vol. 29, no. 6, pp. 515-547. 1995.
Patterson, Bruce D. "Evolving Eden: an Illustrated Guide to the Evolution of the African Large Mammal Fauna" Journal of Mammalogy. Volume 86, Issue 5 (October 2005) pp. 1052-1053
Su,D, and Harrison, T. "The paleoecology of the Upper Laetolil Beds at Laetoli: A reconsideration of the large mammal evidence." Hominin Environments in the East African Pliocene: An Assessment of the Faunal Evidence. Springer Netherlands, 2007, pp. 279-313
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Wow, that was excellent, Brian.
Wow. Thanks so much for clearing all that up, Brian. Hell, even the "Walking with Beasts" episode with hominids subscribed to the savannah hypothesis, so I assumed such a model was still widely used. Looks like things are not so simple, though, as is usual when looking at evolutionary change! This is actually very exciting, and I can't wait to do some research on the topic.
So it seems. Not that I expect this to leave tracks in the science later, but just for kicks:
I stumbled on < href="http://www.plosone.org/article/fetchArticle.action?articleURI=info:doi/10.1371/journal.pone.0001019">one wild data set/hypothesis over mammals and especially primates many morphologies of the lumbar transverse process the other day:
IIRC, the press release refers to the the repositioning as possibly a developmental reversal (with an interesting illustration in figure 14 in the paper - looks like complete reversal) that forced the victims to bipedalism, AFAIU placing some apes as independently deriving older LTP morphology. And claims 4 cases of bipedalism older than afarensis.
Far out.
The missing link to the paper.
Hooray for PLoS! I'll be sure to read this one in more detail. I figured that the author of the paper would be Aaron Filler as he's been championing Morotopithecus lately (he had another paper positing Morotopithecus as one of the transitional bipeds and has recently published a popular books called Upright about his ideas, although I have yet to read it). Maybe I'm wrong, being that I haven't yet read the paper, but it sounds like the reversal being the direct causation of bipedalism is in line with a lot of the recent "natural selection is not a primary mechanism of evolution" stuff we've been hearing about lately (I'll write something up later today about someone at Rutgers who holds such a view). Perhaps Filler is on to something, though, although I haven't seen much reaction to his ideas thus far.
Was it good for you too? :-P Being layman and still browsing around haphazardly I suspected alternative hypotheses were a dime a dozen. (So I can believe the "haven't seen much reaction".) Thanks for the book reference!
I will read your posts on proposals for alternative primary mechanisms in evolution with interest.
Indeed, ideas about the origins of bipedalism are legion, most of them (as far as I'm concerned) generally unsatisfactory. Perhaps this will change as we get a better idea of the animals that were ancestral to hominids and the changing ecology in Africa where they lived, but for now it seems that the savanna is still entrenched as the setting in which we evolved. There are some ideas that aren't dependent on a grassland lifestyle, though, like one where apes stood up to pull down branches in order to reach fruits (there's another book called "Upright" that puts forth this idea, although it's not where it came from), but they don't seem quite right either. Indeed, as much as I'm interested in anthro there is a lot of controversy and a lot of big egos and a lot of subjective ideas being that it's the study of our close evolutionary relatives so I don't know if I'd ever want to be more than a spectator/commentator, but I hope that more attention will be paid to ecology and the apes that gave rise to hominids as I think some of the answers we're looking for will come from such research. The same goes for bird evolution; Cretaceous feathered dinosaurs and early birds and great, but as scientists like Peter Dodson has noted, we need to be looking at deposits contemporary with and older than Archaeopteryx if we're going to get a clear picture of the evolution of birds and maniraptoran dinosaurs.
Oh, and before I forget, I will write on the proposed alternatives to natural selection as the driving force behind evolution when I can, although it is difficult to get any straight answers from the people proposing the ideas. I recently saw a speaker at NYU who proposed that speciation events could occur through chromosomal changes and not by isolation/adaptation, but no example was given or any observations discussed (if this has, in fact, been observed). I'll be writing tomorrow about a philosopher from Rutgers who doesn't like natural selection (although he doesn't make a coherent case for what is a primary mechanism for evolution), but in general it seems that there are some discontents among researchers in the field of genetics when it comes to natural selection but they are still remaining cryptic, sometimes stating that they're holding back as to not invoke the ire of "Darwinists" (there's that word again...).
When all other theories fail, Brian, we'll always have the Aquatic Ape Hypothesis to fall back on!
By the synchronicity of the web David MarjanoviÄ mentioned it the day after:
Thanks for the follow up! I agree that this is one of the big problems; if there is some sort of chromosomal change it would have to occur in more than just one individual if it really does represent a speciation event (i.e. reproductive isolation). I don't know if it's possible, but the only way I could imagine a genetic or developmental change that causes an immediate speciation event to create enough individuals with that difference would be some sort of epigenetic change, maybe brought about by diet or some other environmental factor. Such an occurrence is pure speculation, of course, but even if a similar event actually occurred the population would still be subject to natural selection so I don't understand why some scientists have downplayed the role of NS as if it had no effect on evolution. Like I've noted, I'm a bit out of my depth in this area but I'm trying to learn what I can and will write about it as I learn more.