Joined: Jan. 2008
FOXP2 and Human Cognition
Our restless species strives ceaselessly to invent ever more useful devices, improve our social systems, and create new works of art. Our creative ability derives from motor and cognitive flexibility that allows us to form a potentially unbounded number of new words and sentences as well as tools, art, dance forms, and music; it is a fundamental defining attribute of Homo sapiens that presumably derives from a suite of neural capabilities absent or greatly reduced in other species. The archaeological record, however, reveals few signs of creativity earlier than not, vert, similar200,000 years ago in Africa, with a burst of creativity appearing in Homo sapiens during the Upper Paleolithic, not, vert, similar50,000 years ago ([Klein, 1999] and McBrearty and Brooks, 2000 S. McBrearty and A.S. Brooks, J. Hum. Evol. 39 (2000), pp. 453–563. Abstract | PDF (2416 K) | View Record in Scopus | Cited By in Scopus (314)[McBrearty and Brooks, 2000]). Something must have modified the brains of our ancestors in that distant time, the period associated with both the appearance of the immediate ancestors of modern humans and the amino acid substitutions that differentiate the human form of the FOXP2 gene from that of chimpanzees. Now, Enard, Paabo and their colleagues shed new light on the role of the FOXP2 gene on the evolution of human language and cognition (Enard et al., 2009).
They report, in this issue, the results of introducing into mice the human version of the Foxp2 gene. The mice exhibited alterations in ultrasonic vocalizations and exploratory behavior as well as changes in brain dopamine concentrations. The neurological consequences provide an explanation for why human speech, language, and cognitive capacity transcend those of living apes, as well as the cognitive abilities of our distant hominid ancestors that can be inferred from the archaeological record. In mice with a “humanized” Foxp2 gene, the medium spiny neurons of the basal ganglia show increased synaptic plasticity and dendrite length. Such changes enhance the efficiency of neural cortico-basal ganglia circuits, the brain mechanisms that in humans are known to regulate motor control including speech, word recognition, sentence comprehension, recognition of visual forms, mental arithmetic, and other aspects of cognition (Figure 1).
This brings us to the signal achievements of Enard and his colleagues (Enard et al., 2009). The FOXP2 story started with the discovery of a mutation in this gene in an extended family in the UK that resulted in extreme speech motor-control deficits, deficits in language comprehension, and lower scores on standardized intelligence tests. Neuroimaging studies revealed anomalies in basal ganglia morphology and activity. Embryological studies then showed that both the mouse and human versions of this gene modulate development of the basal ganglia and other subcortical structures. Moreover, the two amino acid substitutions that differentiate the human form of FOXP2 from that of chimpanzees occurred and were fixed within the past 200,000 years, the period associated with the appearance of the immediate ancestors of modern humans and Neanderthals. However, it has not been clear whether the behavioral deficits associated with the aberrant missense mutation in the affected family members have any bearing on the effects of the human form of FOXP2 on the brain. With their new study, Enard and coworkers resolve this issue. They demonstrate that the amino acid substitutions that mark the human form of FOXP2 would have played a key role in the evolution of the human brain by increasing synaptic plasticity and dendrite length and connectivity in the basal ganglia.
The proximate “tinkering” logic of evolution has often been pointed out. In a sense, we can view the effects of the human form of FOXP2 as a sort of “tuning” that brought the cortico-striatal circuits that humans inherited from other species to a state of higher efficiency. Synaptic plasticity is the key to how neurons code and process information. Dendrites connect the neuronal map, channeling information between neurons. Neurophysiological texts contain hundreds of references to studies that note the roles of synaptic plasticity and neuronal connectivity in forming new associations and new action patterns—the Hebbian (Hebb, 1949) “computational” processes of the brain that appear to underlie virtually all aspects of cognition.
As is the case for all significant discoveries, the new work addresses seemingly unrelated issues and raises further questions. The earliest surviving hominid fossils that could have had tongues capable of producing fully modern speech date back 50,000 years to the Upper Paleolithic (Lieberman and McCarthy, 2007). In earlier Middle Pleistocene fossils, in which the neck segment is equal to the mouth segment, neck lengths were too short to accommodate a human tongue. Tongue proportions that facilitate speech came at the cost of increasing the risk of choking—the fourth leading cause of accidental death in the U.S. Therefore, a human tongue would be worse than useless unless the hominid in question also had cortico-basal ganglia circuits capable of executing the rapid, complex motor gestures that are necessary to produce articulate speech. The presence of a human tongue in Upper Paleolithic hominids thus serves as an index for the presence of these neural circuits. But as Enard et al., 2009 W. Enard, S. Gehre, K. Hammerschmidt, S.M. Hölter, T. Blass, M. Somel, M.K. Brückner, C. Schreiweis, C. Winter and R. Sohr et al., Cell (2009) this issue.Enard et al. (2009) show, cortico-basal ganglia circuits could have evolved before the appearance of the modern human tongue, explaining the presence of some Upper Paleolithic artifacts in Africa >50,000 years ago.
Finally, these results argue against Noam Chomsky's views concerning the neural bases of human language. In all versions of Chomskian theory, the central claim is that humans possess a species-specific, innate, neural “organ,” devoted to language and language alone. Language in Chomsky's theories, moreover, is equated with syntax, the means by which distinctions in meaning are conveyed in a sentence. Cortico-basal ganglia circuits clearly are involved in sentence comprehension, but enhanced human cortico-basal ganglia circuit efficiency clearly would be expressed in cognitive acts beyond language and motor control. With the study by Enard and his colleagues, we have reached a new milestone in the journey toward understanding the evolution of human cognition.
Enard et al., A Humanized Version of Foxp2 Affects Cortico-Basal Ganglia Circuits in Mice
Cell, Volume 137, Issue 5, 29 May 2009, Pages 961-971
Short video summary of Enard's and Pääbo's results
ETA: The original article is open access. If the above link isn't working try this one.
"Random mutations, if they are truly random, will affect, and potentially damage, any aspect of the organism, [...]
Thus, a realistic [computer] simulation [of evolution] would allow the program, OS, and hardware to be affected in a random fashion." GilDodgen, Frilly shirt owner