Corredores de fondo (entrevista a Daniel Lieberman)

En la larga carrera de la evolución la resistencia se revela de mayor importancia que la potencia. Algunas especies muy especializadas pueden tener un notable éxito mientras su nicho no se vea afectado por cambios ambientales. Encajan perfectamente en la naturaleza, pareciendo estar hechas para el sitio que ocupan en ella. Ahora bien, cualquier pequeño cambio en su ambiente propicio puede borrarlas del mapa y de la historia naturales en poco tiempo. Otras especies, denominadas oportunistas, tienen un comportamiento, una anatomía y una fisiología más adaptables, y pueden prosperar en diversos ambientes. Para que estas desaparezcan se requiere algún tipo de catástrofe ambiental.

De entre las especies oportunistas la nuestra ha resultado ser la más exitosa. A partir de nuestros ancestros arbóreos hemos ido evolucionando hacia un bipedalismo activo. Y hubo un tiempo, al parecer, en el que correr largas distancias sin desfallecer nos otorgó una ventaja adaptativa decisiva para sobrevivir en sabanas abiertas. O así, al menos, lo sugirieron en antropólogo de Harvard Daniel E. Lieberman y en biólogo de Utah Dennis Bramble en un artículo en Nature.

Ya en su momento señalamos cómo una anatomía y una fisiología diseñadas para la carrera de resistencia y unas habilidades cognitivas adecuadas para seguir pistas podrían haberse combinado para dar lugar al más inteligente e implacable cazador. La capacidad de refrescarnos a través del sudor, la verticalidad que reduce la cantidad de cuerpo expuesta al sol, así como la visión frontal y a larga distancia desde una posición elevada y una colocación de huesos, músculos adecuada para la carrera, entre otras cosas, nos permitieron carroñear y cazar con cierta ventaja especial sobre competidores y presas. Así, unos primates relativamente débiles, de carrera lenta y carentes de otras defensas que la que su inteligencia y su trabajo en equipo les proporcionasen, sobrevivieron en el ambiente hostil de la África ancestral.

Daniel Lieberman ha trabajado en el amplio campo de la evolución humana, como su padre, Philip Lieberman. Sin embargo sus carreras han tomado un curso distinto. Dicho de forma simple, Daniel sigue las pistas de lo que nos hizo humanos anatómicamente, Philip investiga en lo que nos hizo humanos espiritualmente, analizando el lenguaje. Ambas carreras convergen en la caza y captura de nuestra esencia, de lo que nos hace únicos.

1.- In which degree the ability to run long distances contributed to make us human? What was its importance for hunting? Which came first, hunting or running?

It is impossible to be sure, but I think that these behaviors co-evolved possibly after humans started to scavenge. Between 3 and 2 million years ago the savannas in Africa started opening up and some hominins (the genus Homo) started to eat higher quality foods such as meat, marrow, and brains. Maybe this started first as scavenging, but then hominins also started hunting. However, at this time, projectile weapons did not yet exist (the spearhead was invented less than 300,000 years ago), so I hypothesize that early hunting was made possible by the ability to run long distances in the heat, driving animals into hyperthermia. This is possible because quadrupeds cannot gallop and pant (panting is how quadrupeds cool down), but humans cool by sweating. If a human makes a quadruped gallop for a long time (>15-20 minutes) in the heat, it will get heat stroke, at which point one can kill it easily without any sophisticated technology or risk of injury. So endurance running combined with tracking may have enabled Homo erectus to hunt millions of years before we invented the bow and arrow and other such technologies.

2.- When did we freed our hands? When did they began to be used for making tools? When did they light the first fire?

Hands became freed from a locomotor role when we became bipeds, probably 5-7 million years ago. Chimpanzees make simple tools, so early hominins probaby also did so as well. Stone tools started to be produced about 2.6 million years ago. The oldest evidence for fire is controversial, but it does not probably appear until 750,000 years ago, and does not become very common until less than 400,000 years ago.

3.-What do we know today about homo florensis? Its reduced body size point to a long stay hominid adapted to the limited resources of a island? or, else are there other hipothesis about the skulls found?

This is too complex a question to answer here! The most likely hypothesis is that its small size was a form of endemic (insular) dwarfing. This might occur because of limited resources on an island, but may also occur because islands tend not to have predators removing any advantage of being big. Alternative hypotheses are that H. floresiensis was small because it was descended from H. habilis (which was much smaller than H erectus), or that they are a pathological modern humans. I suspected it is a dwarfed H. erectus, but more data rare needed to prove this.

4) What do you think about the hypothesis that the evolution of bipedalism occurred in trees? Would this not lead to questioning the idea that the bipedalism occurred in response to a tree environment that was in decline?

Why hominins started being bipedal is very hard to pin down. There was certainly an initial postural phase (chimps can and do stand bipedally for all sorts of reasons including feeding) but I don’t think this is a compelling argument for the morphological shift to habitual bipedalism.A more likely explanation is that bipedalism is much more economical than knuckle-walking (see Sockol et al., 2007 in PNAS). As the rain forests started to disperse at the end of the miocene, apes living at the edge of these forests may have undergone strong selective pressure to travel long distances between patches of trees. Bipedalism might have been selected for because human (and probably australopith) bipedal gaits are 25% less costly than knuckle-walking.

5) What would you say is our oldest ancestor? How many species of hominid is revealing the fossil record? How complete is, at the date, the jigsaw of our evolutionary past?

Our oldest known ancestor (that is something more closely related to us than to a chimpanzee) is Sahelanthropus tchadensis, which lived at least 7 million years ago. The fossil record for early human evolution, however is very sparse, and it doesn’t start to get very rich until about 4 million years ago. Even then, there are many periods and regions for which we have limited data.

6) How do you think language arise? Do you think that Homo sapiens sapiens is the only one who even had this faculty?

I don’t know the answer to this question (nor does anyone else). I would assume than language evolved, like other complex traits gradually with many intermeidate steps. At what point you start calling it language is not easy to determine (nor very useful). Further, I would be surprised if language and brian size weren’t somehow related. So H. erectus probably had some kind of language.

7) What evolutionary pressures would you say that caused the spectacular growth of the brain in hominids? How was that possible?

Again, this is an impossibly complex question! Big brains are VERY costly (a human brain costs about 25% of our metabolism), and they entail all kinds of problems (birth, locomotion, etc). They would only be selected for if they had some advantage, which is presumably cognitive (language, planning, etc). I think the more interesting question is what are the constraints in brain size. Why didn’t big brains evolve more often and/or earlier?

What are you working on now? What is your highest intellectual challenge? What is the mistery that you would dream to uncover?

I am interested in why and how the human body looks the way it does and what lessons this has for how we use our bodies today. Right now I am particularly interested in how we use our bodies to run long distances and how and when this happened. Among the problems I am working on is how the foot works during running when one is barefoot rather than wearing shoes.