"Savanna" does not mean "Desert, grassland, woodland, and forest"

Yesterday was a little light on posts as I was in transit for most of the day to a lecture at NYU by Kevin Hunt of Indiana University called "The inferred forest home of the earliest hominins: Firm foundation or house of cards?" The talk was much more narrow in scope than what it might sound like from the title however, and there were good points and relatively weak points made throughout the presentation. Here I present a few thoughts from notes I took during the lecture and conversations with the Rutgers professors and grad students that also attended after the lecture;

When I hear the term "savanna" I think of an open grassland with very little tree cover, inhabited by lions, giraffes, and various grazing bovids, a woodland (40-80% tree cover but not a entirely closed canopy) and a forest (closed, interlocking canopy of large trees) being quite different. In the past, however, some researchers have played fast and loose with the terms savanna, grassland (which I take as being synonymous with savanna), woodland, and forest, the term "savanna" sometimes being used to describe everything from desert to forest. Variations like "open forest" (= woodland?) or "closed woodland" (= forest?) have further confused the issue, and Hunt was spot on in identifying that there's a "failure to communicate" about what certain faunal assemblages might indicate and paleoecology might indicate at a site. He primarily drew his examples from "Mio-Pliocene mammals from the Middle Awash, Ethiopia" by Haile-Selassie et al., noting that the fauna described in the paper (although with other data sources) appear to indicate a grassland setting while the paper says that it was a "wooded habitat." Was there a disconnect between the people who were doing the faunal analysis and the lead authors, or are we just not being rigorous enough with our terms?

I applaud the fact that Hunt wants to create a sort of standardized listing of habitat types and characteristics, but as some people in the audience pointed out during the Q&A the fossil record gives us no clue as to whether an area had between 40 and 80% canopy cover; whatever definitions are generated have to be consonant with the types of data retrieved from paleontological sites in order to make sense. Beyond that, when we say a habitat was a mosaic of several habitat types it might help to get an idea of the relationships of the habitats to each other. Was the forest bounded by woodland that transitioned into grassland? Do we have any idea of the proportions of each (was it mostly grassland with narrow riverine forests?). A little more resolution is needed, but this won't be accomplished via one paper and hopefully it'll kick off a bigger debate about the usage of terms.

All of the above is significant because it directly relates to our ancient relatives in the region, Ardipithecus ramidus being specifically mentioned here. The choice of Ardipithecus is a little problematic, however, as some see it as being more closely related to chimps and there's still Orrorin tugenensis from Kenya and Sahelanthropus tchadensis from Chad, all of which are somewhat vying for position of being a hominin ancestor most closely related to humans (Orrorin and Sahelanthropus were not mentioned during the talk). Furthermore, evolution works via populations; one species does not evolve towards one end across an entire range at the same rate at the same time, so even if we get a better resolution on one site or habitat for a hominin ancestor it doesn't mean we've answered the question of habitat for all members of that species everywhere. Modern chimpanzees, for example, live in a variety of habitats, the population at Mt. Assirik in Senegal living in a much more open habitat than the chimpanzees at Gombe or Tai, each having their own cultures and differences.

After the opening about the paleoecology of the Middle Awash, Hunt related the area to his research on chimpanzees in Uganda at Toro-Semliki, the apes living in riverine forests along a number of tributaries, venturing out into the woodlands by avoiding the open savanna as if it were poison. The behavior and habits of these chimpanzees were not directly related back to the Middle Awash for comparison, but it was at least inferred that they might have been living in a similar habitat as Ardipithecus and that there are two kinds of foods that the Semliki chimps eat present in the Middle Awash, one being tamarinds. Things got a little sticky here as Hunt mentioned that he favored a foraging origin for bipedalism (not unlike Jolly's "Seed Eaters" hypothesis) where early hominins or their ancestors stood bipedally to eat fruit and nuts on the ground and in trees, but I've never particularly liked this hypothesis as the answer for the origin of bipedalism. I don't doubt that early hominins and their direct ancestors stood up to gather food at least occasionally, but I don't see how such behavior provides enough of a selective force for bipdealism to fully develop (how would standing up to pick fruit translate into greater survivorship and/or great reproductive success?) and I think there is much more to the story. Anyway, Hunt mentioned that two fruiting bodies of plants present in Semliki today were present at early hominin sites in the past, attempting to make a connection. Perhaps early hominins and their ancestors at tamarinds, too, but there was a long list of other plants known in the fossil record as well as small vertebrae and invertebrate prey, so while there is this point or correlation I don't know if we can say more than "Maybe they ate this sort of food as well."

As I mentioned before, though, this is an analysis of just one area among many correlated to a modern-day habitat that may be influenced by the burning of grasslands by park rangers to help populations of kob that were previously devastated, so it's hard to make direct connections. Whatever criticisms or questions I might have, though, the main point of the lecture that I agreed with is a need for more rigorous definitions of ecological terms for use in the fossil record and more communication between paleoanthropologists and ecologists, botanists, zoologists, etc. in order to get better resolution on different sites.