Mirror Neurons And Encultured Babies

A little piece in LiveScience about the recent work of Victoria Southgate, at Birkbeck College, London. Using EEG she has documented the employment of mirror neurons in babies 9 months old that fire when babies both watch people reaching for objects or perform the same motor task for themselves. Interestingly, this mirror neuron behaviour proved to be predictive in that it was amenable to training. Once the baby had seen a hand appear from behind a curtain to grab something, it would fire just before the hand grabbed on future trials. Livescience report Southgate making the point that this could lead to accurate responses to others' actions, including interception and underpin the first steps babies make into the social world because it allows them to take part in collaborative activities.

Why Humans Talk And Chimps Don't

It's a fertile time, at the moment, for interesting comparative functional neuroscience! Here an article reports on recent work on Broca's Area, by Chet Sherwood and team. They sectioned and examined Broca's Area from a dozen chimps and discovered that there is no left-right asymmetry in terms of number of neurons and that there were a lot of individual differences in the size, location and symmetry of Broca's Area, rather than a pan-species norm - as exists in humans. Neither was the handedness of the chimps in any way related to symmetry of Broca's (Broca's Area in chimps has been related to the use of hand gestures and tool use). Furthermore, they notice that, while the human brain in total is some 3.6 times larger than the chimp brain, Broca's Area in the human is 6 times larger than in the chimp. It has ballooned in the human lineage.

Precursors To Human Language Neuro-circuitry Found In Macaque

The two main language areas of the human brain - Broca's Area and Wernicke's Area - are connected by a major bundle of fibres called the arcuate fasciculus, which also ramifies extensively in the temporal lobe and in areas adjacent to Broca's Area in the frontal lobe. From what areas of the primate brain did the brain structures necessary for language evolve? Michael Petrides and Deepak Pandya, in this PLoS Biology article, report detailed tracing of pathways in the macaque brain they think are those evolutionary precursors. The detailed forensic work is made possible by the injection of radiographic dyes into the white matter nerve fibres. The dye runs along the network allowing tracing to occur. This is their summary:

"Two distinct cortical areas in the frontal lobe of the human brain, known as Broca’s region, are involved with language production. This region has also been shown to exist in nonhuman primates. In this study, we explored the precise neural connectivity of Broca’s region in macaque monkeys using the autoradiographic method to achieve a level of detail impossible in the human brain. We identified two major streams of connections feeding into Broca’s area: a ventral stream from the temporal region, which includes areas processing auditory, multisensory, and visual information and a dorsal stream originating from the inferior parietal lobule and the adjacent superior temporal sulcus. Our detailed connectivity analysis illuminates the pathways via which posterior cortical areas can interact functionally with Broca’s region, in addition to contributing to an understanding of the evolution of language. We suggest that a fundamental function of Broca’s region is to retrieve information in a controlled strategic way from posterior cortical regions and to translate this information into action. This fundamental function was adapted during evolution of the left hemisphere of the human brain to serve language."

Specifically, what they have found is that one of the component parts of Broca's Area, area 45, and a related ventro-lateral area, is involved with the controlled retrieval and selection from non-verbal memory in the monkey and in the right - or non-dominant hemisphere of the human brain. In the human, this area on the left side of the brain has been co-opted for language. Specifically they say:

"During the evolution of the human brain, these high-level forms of programming (the basic elements of which are already present in the macaque monkey brain) came to include complex syntactical structure (e.g., hierarchical level of control) that is
necessary for language, and which has been argued to be a major contribution of Broca’s region. If we were to extrapolate these arguments on the basis of the present monkey anatomical study, our recording study in the monkey, and our functional neuroimaging studies of the human right hemisphere homologue of Broca’s region, we
could say that a common primate circuitry was adapted, during millions of years of evolution, in the human brain for the strategic retrieval and selection of information from verbal memory (including the mental lexicon) in posterior temporo-parietal
cortical regions by one component of Broca’s region, area 45, and the transformation of this selected conceptual information into gestural/speech acts by the other component of Broca’s region, area 44, via its connections with motor structures, such as the premotor cortex, the basal ganglia, and the rostral inferior parietal lobule. Our suggestion here is simply that an area that served higher control of action in the macaque monkey may have been adapted for the control of complex hierarchical sequences of gestural and vocal action with the evolution of communication leading to human speech."

This neatly links the complex hierarchical welding of phonemes into language with the complex and bewilderingly fast oro-facial movements necessary to articulate language in the larynx. When might this evolutionary process have occurred? The authors note that some degree of leftward asymmetry has been found in the planum temporale (associated with Wernicke's Area) in chimps as has left hemisphere dominance or the oro-facial movements involved in learned communicative vocalizations. Also, handedness has been noted for chimps connected to tool use, suggesting a left hemisphere specialization for control of complex actions involving the right hand. Remarkable asymmetry of Broca's cap has been discovered in endocasts of Homo erectus skulls, suggesting the various precursors were coming into place before manifesting themselves, much more recently, in a language faculty. The authors conclude:

"These findings are consistent with suggestions that specialization for the control of action and gesture may have preceded specialization for language. Note also that our close primate relatives, chimpanzees and bonobos, use arm/hand gestures more flexibly in their natural communication across contexts than facial expressions and vocalizations. The above facts suggest that the use of gestures for early forms of communication may have been an adaptation distinguishing the Hominoidea from other primates, and that the use of vocalization in the form of modern speech emerged much later with the evolution of language in the narrow sense, i.e., a uniquely human adaptation. It is interesting in this respect that the supralaryngeal vocal tract of humans differs significantly from those of other primates, making the human vocal apparatus unique in transmitting information at fast rates."

Thus language appears to be a combination of ancient, modern and unique evolutionary developments in the primate and hominin lineage.

Gene Regulation Is A Vital Part of Human Evolution

In NOT A CHIMP I chronicle the extraordinarily prophetical work of Allan Wilson and Mary-Claire King, a quarter century ago, which argued that mutational changes to the DNA sequence of genes - single nucleotide polymorphisms - alone could not possibly explain the myriad differences they recorded between humans and chimps. Something else had to be going on. They suggested that differences in gene regulation - the activity and timing of activity - of identical or highly similar genes would be found to be important. In those days limited genome technology prevented them from fully exploring this idea but recent work has dramatically supported their theory and shown a host of gene regulation differences between humans and chimps, especially in the brain. Now, a group of scientists which includes Carlos Bustamente and Andrew Clark, from Cornell University, has examined the phenomenon of cis-regulation - changes in DNA in non-coding intronic areas flanking more than 15,000 genes - and found patterns they interpret as a strong effect of both positive and negative selection during hominid history - and an especially strong effect of positive selection upon these regulatory sequences in genes operating in the foetal brain. As they conclude in their summary: "Our results suggest that both positive and negative selection have acted on candidate cis-regulatory regions and that the evolution of non-coding DNA has played an important role throughout hominid evolution."

Are Humans Still Evolving?

The popular view is that humans have ceased to evolve as advances in medical care, better housing and welfare, etc. etc. have leveled the playing field and eradicated the essential variability in humans upon which selection can act. The view that human evolution is grinding to a halt is cogently argued by Prof. Steve Jones, from University College, London, who points out that a former great source of mutational change was the sperm of older men. Cell division during sperm manufacture provides plenty of opportunity for mutational mistakes to occur - and in men over 50 those mistakes are far more numerous. But, says Jones, elderly fathers are rare these days - there is a dearth of aged patriarchs fathering children by multiple wives - and this is allied to far greater survival rates among offspring.

I counter that view in NOT A CHIMP by championing the arguments of John Hawks, Greg Cochran and Henry Harpending, who calculate that human evolution stagnated for much of hominin history but has dramatically speeded up in the last 40,000 years. Also, that, if you take a population geneticists viewpoint there are very many more genomes in circulation around the globe today thanks to human population explosion over the last few millennia - all potential sites for mutation.

Now, as reported in The Scientist, life history evolutionist Stephen Stearns, from Yale University, has got his hands on a wealth of longitudinal medical data collected in the famous Framingham study which has been running since 1948 - several generations' worth. With a number of colleagues he is developing an evolutionary twist to the Framingham data-bank where they are correlating variables like lower cholesterol, lower blood pressure, earlier conception and later menopause, with reproductive success (having controlled for social factors that might influence fertility). They have shown clear changes in the levels of these genetically-controlled traits in only 2 to 3 generations and are now expanding the study to include parameters like high-density lipoproteins, triglycerides and bilirubin, and to look at male reproduction. They are due to publish a detailed review of their findings on contemporary human evolution in a special issue of PNAS later this year.

Human Ancestors Were Not Knuckle-Walkers

This ScienceDaily piece reports on a recent paper by Tracy Kivell and Daniel Schmitt, in PNAS. Despite decades-worth of argument, they say, we have not yet resolved whether or not human bipedalism evolved from a terrestrial knuckle-walking ancestor, or a more generalized arboreal ape ancestor. For the first time, they have minutely compared wrist morphology among a number of primate species and conclude that two different forms of knuckle-walking have independently evolved - an extended wrist posture with bony stops to prevent the wrist from over-bending, in chimps, and a locked 'columnar' stance in gorillas where the arms and wrists extend straight down. The duo point out that there are a number of subtle differences in wrist morphology between humans and chimps which suggest to them that we did not evolve from a chimp-mode knuckle-walker but a more generalized arboreal primate ancestor.