How and why is the human brain different?

1. How and why is the human brain different? • The human species is psychologically and behaviourally unique. • At the level of behavioural ecology, humans are a special species simply by not being confined to a single ecological niche (Bingham, 1999) • Beyond that there is a widespread consensus that the human species has unusual psychological capacities, even though there are wide differences in how these should be described or defined.

2. Assuming unique psychological capacities of some kind, the neuropsychological question is then how these depend on special characteristics of the human brain (see e.g. Deacon, 1997a&b). • Broadly speaking, the main answer to this question is “we don’t know” • but there is a range of attempted answers which are worth exploring. • The why? and how? questions are in fact difficult to separate, but the standard answer to why? would be because human evolution has made the human brain different (Bradshaw, 1997; Moll et al., 2005).

3. This standard evolutionary answer does not give us very much to go on, but “evolutionary psychology” of various kinds is becoming more popular, and there is a fair degree of consensus that present and future genetics will be able to add a great deal of detail to our knowledge of human brain evolution. (See, e.g. Ramus, 2006 and Sikela, 2006 on page 7 of the handout)

4. Millions bottom

5. Millions top “Lucy”

6. Mary leakey footprints Fossilized footprints, discovered by Mary Leakey in Laetoli, Tanzania. They are dated at 3.5 Mbp, and only the “Lucy” species is known from that time, but the imprints look very like modern human imprints.

7. Family Tree

8. Alemseged, Z., et al. (2006). A juvenile early hominin skeleton from Dikika, Ethiopia. Nature, 443(7109), 296-301

9. Alemseged, Z., et al. (2006). A juvenile early hominin skeleton from Dikika, Ethiopia. Nature, 443(7109), 296-301 Dikika is only 4km from where ‘Lucy’ was found (Australopithecus afarensis ) The Dikika specimen, from 3.3m yrs ago was about 3yrs old and probably female. The legs were human-like for bi-pedal walking, but the arms and hands ape-like. The hyoid bone (for the larynx) was also ape-like

10. Napier (1980) [not on handout UCL Library]

11. Napier, ape hands

12. Napier – Screwtop

13. Brain Size • This gives one simple answer to the how? question: the human brain is outstandingly big, given our body size. • However it is clearly not the whole answer, since relating brain-size to body weight is complicated (Deacon, 1997b) and the human gain in behavioural terms is more extensive than would be predicted on size alone. • Genes which may be responsible for primate and human brain expansion are currently being investigated (Ponting & Jackson, 2005; Evans et al., 2006; Pollard et al, 2006; Sikela, 2006; Tang, 2006)

14. Brain Size • To what extent are human capacities what we would expect from a 1,500 primate, or 1,500 mammalian (cf dolphins, elephants) brain? • Or are there special ingredients or “magic bullets” (Elston et al., 2006; Allman et al., 2005) which produce uniquely human capacities? • Or are both the above correct?

15. 1,000,000,000,000 350,000,000,000 100,000,000,000 500,000,000 300,000,000 50,000,000 850,000 250,000 20,000 381 302 Homo sapiens (maybe 1014) Chimpanzee Rhesus monkey Mouse Octopus Stickleback Honey bee Fruitfly Sea slug Thread worm male Thread worm Number of neurons in the nervous system

18. Striedter brain szie In Striedter, G. F. (2005). Principles of brain evolution. and Striedter, G. F. (2006). Precis of Principles of Brain Evolution. Behavioral and Brain Sciences, 29(1), 1-+. [not on paper handout, see intranet]

22. Ponting and Jackson, 2005 • …….recent advances from the cloning of two human genes promise to make inroads in the area .. of brain size evolution. • Microcephalin (MCPH1) and Abnormal spindle-like microcephaly associated (ASPM) are genes mutated in primary microcephaly, in which, the brain is of a size comparable with that of early hominids. • It has been proposed that these genes evolved adaptively with increasing primate brain size. Subsequent studies have lent weight to this hypothesis by showing that both genes have undergone positive selection during great ape evolution. • the evolutionary patterns of all four presently known primary microcephaly genes are consistent with the hypothesis that genes regulating brain size during development might also play a role in brain evolution in primates and especially humans (Evans, 2006; Tang 2006)

23. Lateralization • One additional factor is lateralization. • Although there are some who suspect that human brain lateralization is a development of primate asymmetries (Corballis, 2003), the data on population handedness suggests a very sharp distinction between the degree of handedness observed in other primate populations • (which is zero according to the meta-analysis by Papademetriou et al., 2005)

24. Lateralization figure Lateralization - old

25. Lateralization - new • the human figure is 90% right handedness • which does not correlate completely with language lateralization • but they may be related • And the link between physical lateralization and human specializations may be strengthened • By changing techniques (Buchel et al., 2004; Hutsler, 2003; Sun & Watson, 2006; Sun et al., 2006; Luders et al., 2006) • and theoretical models (Monaghan & Shillcock, 2004)

26. They found that the left arcuate fasculus was larger in right handers

27. Luders, E., et al. (2006). Hemispheric asymmetries in cortical thickness. Cerebral Cortex, 16(8), 1232-1238. • Used MRI on 60 healthy adults. • cortex in the left hemisphere was generally thicker. • the precentral gyrus, middle frontal, anterior temporal and superior parietal lobes were significantly thicker on the left • but the inferior posterior temporal lobe and inferior frontal lobe were thicker on the right. • Asymmetry profiles were similar in both sexes

28. Sun, T., et al. (2006). Genomic and evolutionary analyses of asymmetrically expressed genes in human fetal left and right cerebral cortex. Cerebral Cortex, 16, I18-I25. • they compared gene expression levels in the perisylvian regions of human left-right cortex at fetal weeks 12, 14, and 19 • identified dozens of genes • “identified a subset of genes with human asymmetry humans and altered expression levels between chimps and humans.” • “Our results identify candidate genes involved in the evolution of human cerebral cortical asymmetry.”

29. Sun, T., & Walsh, C. A. (2006). Molecular approaches to brain asymmetry and handedness. Nature Reviews Neuroscience, 7(8), 655-662

30. Sun, T., & Walsh, C. A. (2006). Molecular approaches to brain asymmetry and handedness.

31. Hutsler, 2003 7 autopsies: 50-97 yrs of age

32. Monaghan, P., & Shillcock, R. (2004). Hemispheric asymmetries in cognitive modeling: Connectionist modeling of unilateral visual neglect. Psychological Review, 111(2), 283-308 They claimed that a hemispheric distinction between coarse- coding in the RH and fine- coding in the LH exists at the neuronal level as different sized receptive fields. Simulations with connectionist models with these properties were successful in modeling various tests of unilateral visual neglect

33. nottebohm

35. Brain re-organization: expansion of the frontal lobes • Apart from lateralization, other kinds of re-organization with-in the brain might have been possible within the time-scale of human evolution. • The most popular hypothesis for many decades has been that the human frontal lobes, presumed to be the main site for planning and self-control, have either generally expanded or undergone some more detailed change (Deacon, 1997a&b; Schoenemann et al., 2005).

36. However, careful MRI scanning of different great ape species and comparison with human scans has led to the conclusion that there has been no disproportionate expansion of the human frontal lobes

37. semendeferi1

38. Semendeferi table The Semendeferi et al., (2002) table

39. Brain re-organization: expansion of the frontal lobes The sherwood 2005 • Schoenemann et al.(2005) recently suggested that prefrontal white matter is disproportionately larger in humans than in other primates • but Sherwood et al. (2005) countered that a) the boundary between prefrontal and other cortex is not well defined; and b) that in any case, although the data showed humans having more white matter than the average primate, they did not show a difference between humans and great apes.

40. Brain re-organization: expansion of the frontal lobes? Preuss (2004) suggests that what has happened is that, while the primary motor and sensory areas in the human brain are roughly the same size as those in apes, secondary areas (“association cortex”) has greatly expanded in all the lobes of the human brain.

41. Others have suggested that certain neuronal features of primate and human frontal lobes hold the key to understanding human intelligence: pyramidal cells in prefrontal cortex (Elston et al., 2006) or certain spindle cells in anterior cingulate and fronto-insular cortex (Allman et al. 2005; see back of handout, and p. 7 of handout for abstracts).

42. Fig. 4. Neurolucida tracings of pyramidal (left) and von Economo (right) neurons from Fronto-Insular (a) and Anterior Cingulate Cortex (b). Notice the vertical symmetry and relative sparseness of the VEN dendritic tree. [back of handout; Allman et al., 2005]

43. [Back of handout: Allman et al., 2005]

44. Allman, J. M., Watson, K. K., Tetreault, N. A., & Hakeem, A. Y. (2005). Intuition and autism: a possible role for Von Economo neurons. Trends in Cognitive Sciences, 9(8), 367-373. [p 7 of handout] Von Economo neurons (VENs) are a recently evolved cell type which may be involved in the fast intuitive assessment of complex situations. As such, they could be part of the circuitry supporting human social networks. We propose that the VENs relay an output of fronto-insular and anterior cingulate cortex to the parts of frontal and temporal cortex associated with theory-of-mind, where fast intuitions are melded with slower, deliberative judgments. The VENs emerge mainly after birth and increase in number until age 4 yrs. We propose that in autism spectrum disorders the VENs fail to develop normally, and that this failure might be partially responsible for the associated social disabilities that result from faulty intuition.

45. Elston, G. N., et al. (2006). Specializations of the granular prefrontal cortex of primates: Implications for cognitive processing. Anatomical Record Part a-Discoveries in Molecular Cellular and Evolutionary Biology, 288A(1), 26-35. [Abstract on p. 7 of handout] The biological underpinnings of human intelligence remain enigmatic. ….we demonstrate that the basic neuronal building block of the cerebral cortex, the pyramidal cell, is characterized by marked differences in structure among primate species……. …..pyramidal cells in the granular prefrontal cortex of humans had a disproportionately high number of spines… …. the highly branched, spinous neurons in the human granular prefrontal cortex (gPFC) may be a key component of human intelligence.

47. Kaas, J. H. (2005). From mice to men: the evolution of the large, complex human brain. Journal of Biosciences, 30(2), 155-165. [not on handout]

48. Elston et al 2006, abstract on p. 7 of handout. Note the absence of comparisons either with apes or with association cortex in other lobes.

49. Granular prefrontal cortex is represented by stipple. bar 2 cm for human and 1 cm for other species. (another paper from the Elston Lab) However, since even the galago, a prosimian, has got some prefrontal cortex, it is another case where it looks as though the human brain is what would be expected in a typical primate brain expanded to 1500 ccs

50. The galago or bushbaby, a noctural cat-sized tree dweller, has brain organisation which is recognizable in terms of the larger rhesus monkey version.