Confirman Que el Ancestro Común Universal Existió

Hace más de 150 años, Darwin propuso la teoría de la ascendencia común universal, que enlaza todas las formas de vida por una herencia genética compartida, desde los microorganismos unicelulares hasta los seres humanos. Hasta hace poco, esta teoría había estado más allá del alcance de una comprobación formal. Sin embargo, ahora se ha conseguido por fin presentar los resultados de la primera comprobación cuantitativa a gran escala de esta famosa teoría que sustenta a buena parte de la biología evolutiva moderna.


Los resultados del estudio confirman que Darwin estaba en lo cierto desde el principio. En su libro de 1859, "El Origen de las Especies" ("On the Origin of Species"), propuso que "todos los seres orgánicos que han vivido en la Tierra han descendido de alguna forma primordial". Durante el último siglo y medio, ha crecido de manera constante la cantidad de evidencias cualitativas de esta teoría, gracias a las numerosas y sorprendentes formas de transición halladas en el registro fósil, y también a la identificación inequívoca de similitudes biológicas fundamentales a escala molecular.

Sin embargo, recientemente, ha crecido el escepticismo entre algunos biólogos evolutivos quienes han planteado que quizá las relaciones evolutivas entre los organismos vivos no quedan mejor descritas por un único “árbol genealógico evolutivo” sino más bien por múltiples árboles interconectados en una "red genealógica evolutiva".
Hay pruebas moleculares, obtenidas recientemente, que indican que la vida primordial pudo haber sido sometida a una desenfrenada transferencia horizontal de genes, como ocurre con frecuencia hoy en día cuando los organismos unicelulares intercambian genes utilizando otros mecanismos de reproducción que no son los usuales. En ese caso, según argumentan algunos científicos, las primeras relaciones evolutivas fueron semejantes a una red, por lo que es posible que las formas de vida evolucionasen a partir de varios ancestros en vez de sólo uno.

Según el bioquímico Douglas Theobald, eso realmente no importa. Podría ser que la vida se hubiera originado varias veces y de manera independiente unas de otras, algo que la teoría del ancestro común universal permite. Si es así, la teoría sostiene que se produjo un cuello de botella en la evolución, y los descendientes de sólo uno de los grupos primigenios independientes sobrevivieron hasta llegar a nuestros días.

O, por otra parte, también existe la posibilidad de que poblaciones separadas se mezclaran y combinasen con el paso del tiempo, mediante el intercambio de suficientes genes, hasta desembocar en una especie que se convirtió en el ancestro común de todas las formas de vida actuales de la Tierra. De un modo u otro, todas las formas de vida aún estarían genéticamente relacionadas.

Valiéndose de potentes herramientas informáticas y aplicando estadística bayesiana, Theobald ha descubierto que las evidencias respaldan abrumadoramente a la teoría de Darwin de la ascendencia común universal, con independencia del papel de la transferencia horizontal de genes o de los posibles orígenes múltiples de la vida. La ascendencia común universal es millones de veces más probable que cualquier teoría sobre múltiples linajes independientes.
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Anthropology, Primatology, and the Definition of Culture: Reply to Sperber

Category: Anthropology • Great Apes • Human Evolution • Philosophy of Science • Primatology • Research Blogging
Posted on: June 15, 2010 8:30 AM, by EMJ

Chimpanzees have culture (or not) depending on your definition.
Image: Irish Wildcat / Creative Commons

Author's Note: The following is an expansion on my reply to anthropologist Dan Sperber on the PLoS ONE article "Prestige Affects Cultural Learning in Chimpanzees."
Culture is like art or pornography, it's hard for people to define but everyone knows it when they see it. Cultural anthropologists have long struggled to develop a consistent definition of the very thing that they study, a problem that has resulted in bitter arguments between scholars that, to an outsider, may seem as esoteric as church doctrinal disputes over how many angels can sit upon the point of a needle.
In his 1959 book The Evolution of Culture anthropologist Leslie White famously defined culture as "the extra-somatic means of adaptation for the human organism." His goal was to bring some consistency to a field that had 164 separate definitions of "culture" being used interchangeably in the anthropological literature (which, predictably, made cross-cultural comparisons challenging at best). Today, this view has expanded beyond the human animal and a widely accepted definition is from Peter Richerson and Robert Boyd's celebrated work Not By Genes Alone: How Culture Transformed Human Evolution:

Culture is information capable of affecting individuals' behavior that they acquire from other members of their species through teaching, imitation, and other forms of social transmission. By information, we mean any kind of mental state, conscious or not, that is
acquired or modified by social learning, and affects behavior.

Earlier I reported on a new study in PLoS ONE by Victoria Horner, Darby Proctor, Kristin E. Bonnie, Andrew Whiten, and Frans de Waal that found chimpanzees will adopt novel behaviors after watching them performed by high-ranking members of their group. The authors concluded that these findings demonstrate "prestige-based cultural transmission" for the first time in nonhuman animals. Their results were consistent with Richerson and Boyd's definition of culture as well as their argument that:

[N]atural selection has shaped the psychology of social learning so that we are predisposed to imitate people with prestige and material well-being. . . [M]any phenomena, ranging from maladaptive fads and fashions to group-functional religious beliefs to symbolically marked boundaries between groups, might result from the properties of prestige bias.

However, French anthropologist Dan Sperber (Research Director at the Jean Nicod Institute, CNRS and 2009 recipient of the Claude Levi-Strauss Prize in Social Science) has recently challenged these findings in chimpanzees and insists that it does not represent cultural transmission at all. In a critique, following from his work in linguistic anthropology, he suggests that humans alone are capable of culture. However, just like in anthropology's past, his conclusions rest on the definition that he prefers to use.

Responding to the study at PLoS ONE Sperber states:

Talking of prestige among the chimpanzees, who don't gossip about one another, extends the idea of prestige to that of being seen as superior. This then supports the prestige bias thesis only if, in that thesis, prestige does not mean more than this. In other words what it supports is at best a diluted thesis where the ordinary or the sociological notion of prestige plays no role at all.

In other words, because chimpanzees don't use language to gossip about one another, it doesn't fit the definition of prestige and therefore isn't a genuine example of cultural transmission. Sperber then goes on to state that the variables of age and rank were not properly accounted for and argues that the third criteria of prestige in this study (chimps with previous experience introducing novel behaviors) brings with it a "risk of circularity":

We don't know what factor had helped an individual chimp to introduce novel behaviors in the past (imagine, for the sake of argument that it was something about her smell), and so it can be that same factor that explains her doing it successfully again.

There are a number of things wrong with this argument (some of which Horner et al. have responded to). First off, Sperber's insistence that prestige is limited to populations who "gossip about one another" (i.e. humans) doesn't take into account the amount of information that can be conveyed non-verbally. Chimpanzees are highly social and utilize grooming in much the same way humans use conversation. British anthropologist Robin Dunbar measured the number of individuals in the average human clique and estimated that human gossip was almost three times as efficient a bonding mechanism as chimpanzee grooming (he later expanded this hypothesis in his book Grooming, Gossip, and the Evolution of Language). His argument was that human gossip evolved out of the same social utility that grooming served in our common ancestors with chimpanzees. However, in a reanalysis of Dunbar's data by Japanese primatologist Michio Nakamura, he estimated that:

Chimpanzees can obtain about the same efficiency as humans in terms of quantity of social interactions because their grooming is often mutual and polyadic [involving three or more individuals].

In other words, the amount of social bonding that Sperber insists can only occur through gossip can be achieved nearly as well through nonverbal grooming behavior.
Second, it's difficult to understand how Sperber could object that Horner et al. didn't separate the variables of age and rank considering that research on prestige in human societies doesn't separate these variables either. Both are intertwined in the anthropological literature and contribute to the prestige of an individual within a given society.
For example, anthropologist Allyn Stearman wrote of the Yuquí foragers in eastern Bolivia:

The Yuquí concept of leadership and prestige . . . consists of (1) being saya [upper caste], (2) being a good hunter and therefore provider of meat to the band, (3) having senior status based on age (but only relative to the ages of the rest of the band), and (4) possessing a certain charisma in terms of an aggressive personality and ability to deal with peers.

Elsie Begler, in her analysis of egalitarian societies, likewise found age and rank to be intertwined:

Age almost invariably provides the basis for a system of ranked statuses, whether appearing as formal age-grades, or simply being recognized informally as stages through which a person passes in the course of his/her life.

Finally, Joe Henrich and Francisco Gil-White, the same researchers cited by Horner et al. as an example of denying prestige in nonhuman animals, likewise linked age and rank together in their definition:

Age is a proxy for skill/knowledge/success; the longer someone has lived, the more and better skills/knowledge he/she has likely accumulated. Simply living longer is a complex "skill" with acquirable components. Deference toward elders allows proximity and thereby promotes the acquisition of useful information. This reasoning predicts a general correlation between age and prestige, and also that elderly individuals will maintain their status well past their prime.

If Sperber accepts this multifaceted understanding of prestige for human societies it would be hypocritical to object when the same standard is applied to nonhumans.
Furthermore, the suggestion that there was "circularity" in the study because Model A in both groups had previously been observed introducing novel behaviors can't be taken as a serious objection. The purpose of the experiment was to determine whether novel behaviors were adopted because of prestige or because they were more effective. In the experiment both high-ranking Model A and low-ranking Model B received a food reward after placing a token in their respective containers, so the only reason to follow one versus the other was the social prestige of the model. What Sperber calls introducing bias was actually the variable the researchers were interested in studying.
These methodological critiques suggest that Sperber may not have a great deal of familiarity with primates or the primate literature. An additional example of this is his suggestion that "something about her smell" may have influenced the decision to follow Model A versus Model B. Atsushi Matsui and colleages have recently shown that there are no significant differences in the number of functional olfactory receptor genes between marmosets, macaques, and the hominoids (see Jerry Coyne's blog for a review). Humans and chimpanzees have nearly identical numbers of intact or truncated OR genes (396 in humans compared to 399 in chimps) and both species use olfaction in largely the same way. Furthermore, as Horner et al. reported, there were no threat displays by the chimpanzee models nor unusual vocalizations during the experiment that would have influenced other members of the group. The researchers further controlled for the color and appearance of the containers by using two separate groups and reversing the container used in each. In both groups it was the prestigious chimp alone who influenced others to follow her lead.
Sperber has shown this unfamiliarity with the nonhuman literature before. In his critique of Richerson and Boyd's Not By Genes Alone he made this rather shocking claim:

In non-human animals, relatively little information if any is acquired by social learning. Humans on the other hand owe much of their information to others.

This is profoundly wrong. There isn't space to do an adequate literature review of the many examples of social learning in nonhuman animals (but see the edited volume Social Learning in Animals for an excellent overview). However considering that Sperber's area of expertise is cultural linguistics I will cite a few sources that he ought to be made aware of: "Vocal Learning in Mammals" (Janik and Slater, 1998), "The Evolution of Vocal Learning Systems in Birds" (Farries and Perkel, 1997), "Social Communication in Whales and Dolphins" (Tyack, 2009), "The Different Roles of Social Learning in Vocal Communication" (Janik and Slater, 2000), "Social Processes in Communication and Cognition in Callitrichid Monkeys: A Review" (Snowdon, 2001), "How Can We Know the Dancer From the Dance?: The Dynamic Nature of African Great Ape Social Communication" (King, 2003), "Ethological Studies of Chimpanzee Vocal Behavior" (Mitani and Wrangham, 1996), "Geographic Variation in the Calls of Wild Chimpanzees: A Reassessment" (Mitani, Hunley, and Murdoch, 1999), and "Dialects in Wild Chimpanzees?" (Mitani et al., 2005). This is, of course, just a partial sample of studies that identified social learning in vocal communication. Including other forms of social behavior would result in a great deal more.
Given all of this, I don't think the primary disagreement Sperber had with Horner et al.'s paper was its methodology. Rather, I suspect that his objection comes from the definition of culture that he prefers and the kind of cultural anthropology that he studies. Sperber is perhaps best known for his work with linguist Deirdre Wilson on the mechanisms of communication, what he refers to as the "epidemiology of representations." These microprocesses involve the subtleties of human language that, by definition, would exclude any species that doesn't have the linguistic complexity of humans.
It is from this perspective that he challenged the definition of culture as "information capable of affecting individuals' behavior" presented by Richerson and Boyd and insisted that:

To explain culture so understood, the object of study must be the overall flow of information among humans, through its mental and public implementations; the question that must be answered is what causes some of [sic] causal chains to extend more than others in time and space and to stabilize better than others the contents they vehiculate. For this, the study of culture must be embedded in a more general epidemiology of representations and practices that attends--as does medical epidemiology--to the complexities of both individual and ecological mechanisms.

I admire a good deal of what Sperber has to offer in seeking to understand the complexities of human culture (and we share a common objection to Richard Dawkins' meme theory), but in this case he appears to be using an unnecessarily narrow definition that would restrict any species but humans from demonstrating cultural behavior.
References:
Horner, V., Proctor, D., Bonnie, K., Whiten, A., & de Waal, F. (2010). Prestige Affects Cultural Learning in Chimpanzees PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010625
Richerson, P. & Boyd, R. (2005). Not By Genes Alone: How Culture Transformed Human Evolution. University of Chicago Press: Chicago.
Dunbar, R., Duncan, N., & Nettle, D. (1995). Size and Structure of Freely Forming Conversational Groups Human Nature, 6 (1), 67-78. DOI: 10.1007/BF02734136
Nakamura, M. (2000). Is Human Conversation More Efficient Than Chimpanzee Grooming? Human Nature, 11 (3), 281-297. DOI: 10.1007/s12110-000-1014-2
Stearman, A. (1989). Yuquí Foragers in the Bolivian Amazon: Subsistence Strategies, Prestige, and Leadership in an Acculturating Society Journal of Anthropological Research, 45 (2), 219-244. Permalink
Begler, E. (1978). Sex, Status, and Authority in Egalitarian Society American Anthropologist, 80 (3), 571-588. Permalink
Henrich, J. (2001). The Evolution of Prestige: Freely Conferred Deference as a Mechanism for Enhancing the Benefits of Cultural Transmission Evolution and Human Behavior, 22 (3), 165-196. DOI: 10.1016/S1090-5138(00)00071-4
Matsui, A., Go, Y., & Niimura, Y. (2010). Degeneration of Olfactory Receptor Gene Repertories in Primates: No Direct Link to Full Trichromatic Vision Molecular Biology and Evolution, 27 (5), 1192-1200. DOI: 10.1093/molbev/msq003
Sperber, D. & Claidière, N. (2008). Defining and Explaining Culture (Comments on Richerson and Boyd, Not By Genes Alone) Biology and Philosophy 23, 283-292. Permalink

LA CONFIANZA SUBCONSCIENTE QUE INSPIRAN LAS MUJERES

Viernes, 11 de Junio de 2010 08:06

Un breve y leve contacto físico ejercido por una mujer puede hacer bajar la guardia. Esa es la conclusión de un nuevo estudio. En el mismo, se comprobó que si un experimentador de sexo femenino posaba su mano cariñosamente sobre el hombro de la persona, de manera leve y suave, los sujetos así tratados decidían luego arriesgar más dinero que si simplemente les hablaba, o si era un hombre quien ejercía el contacto físico. Los investigadores piensan que esto es una consecuencia derivada del modo en que las madres usan el contacto físico para hacer que sus bebés se sientan seguros. Cuando somos niños, recibimos mucho contacto físico de nuestras madres. Esto crea una sensación de apego y unión que hace que un bebé se sienta seguro. Y a su vez, esto contribuye a reforzar el sentido de aventura del niño: Está más dispuesto a correr los riesgos de explorar contextos no familiares y situaciones extrañas, porque sabe que su madre le protegerá. Jonathan Levav, de la Universidad de Columbia, y Jennifer J. Argo, de la Universidad de Alberta, querían conocer qué ocurre cuando crecen esos bebés: ¿El contacto físico también afecta a cuán dispuestos están los adultos a correr riesgos? En el estudio, se comprobó hasta qué punto los participantes estaban dispuestos a correr riesgos, como invertir dinero o apostarlo. Cuando comenzaron el experimento, los participantes fueron saludados de maneras diferentes: por un experimentador de sexo femenino o de sexo masculino. Y con un suave y alentador toque en el hombro, o con un apretón de manos, o con ningún contacto físico. Al final del experimento, los participantes respondieron a cuestionarios mediante los que evaluar cuán seguros se sentían. Los investigadores encontraron que los participantes que fueron tocados se sentían más seguros, y además corrieron riesgos mayores, que quienes no lo fueron. Pero esto sólo se cumplía siempre y cuando hubieran sido tocados por una mujer. Si era un hombre quien les tocaba, ese efecto no se producía. Además, el efecto causado por las mujeres era más fuerte con un toque en la espalda que con un apretón de manos. Los resultados sugieren que el contacto físico ejercido por una mujer surte efecto igualmente en adultos que en niños: les hace sentir más seguros y más dispuestos a correr riesgos. Scitech News