Evolution of the human brain

Bipedal Lucy with chimp size brain

bipedalism is efficient

Several size jumps on the way to the modern brain

Probably associated with greater tool use, language and social cognition including a broad range of cultural developments and diet. Recently Dunbar (2007) extended his research to compare pairbonds and brain size. Studies on brain size allometry like Dunbar's and behavioral comparisons among hominoids supports the idea that social complexity requires and thus selects for larger brains.

 

The human brain grows much larger than allometrically predicted for a primate of our body size. (Recall dicussion of neoteny, paedomorphosis, and heterochrony.)

Mosaic changes enroute to the human brain

comparative brains (brain museum) and slide.

(homunculus from EHE)

Human brain features

We have a mammal brain modified for primates (more vision, less olfaction) and our own adaptations (social cognition, language). Human modifications include PFC and neocortex size, more convolutions (cortical folding) greater spinal cord cross-sectional area, and neocortical white matter (axons) increased relatively more than gray matter (neurons) and a memory unit (temporal lobes incl hippocampus) to support complex social structure, culture and language. There are also needs in controlling complex movements in hand and vocal tract.

 

Limber's 'brain as automobile motor" metaphor-- both need fuel, oxygen, waste exhaust, and radiators for cooling. They also need a fuel pump and something like a throttle control 'speed'. The brain needs a mechanism for directing these resources where and when needed.

 

Rilling, J. K., & Insel, T. R. (1999). The primate neocortex in comparative perspective using magnetic resonance imaging. Journal of Human Evolution, 37, 191-223.

Overall size matters!

While primate brains don't differ much? on numbers of neurons, (for more detail) the human brain is 3 to 4 times larger than expected for a primate with our typical body size. (Recall allometric studies on this.) Why should size matter then? It is a debated* question but one plausible answer is that larger size allows greater synapses among neurons -- more and richer interconnections (relatively more white matter?). And it is believed that synapses "contain" our knowledge. So with 100 billion neurons and an estimated 1000 synapses each, there are about 100 trillion synapses possible (1 followed by 14 zeros).

(*Striedter, G. F. (2004). Principles of brain evolution. Sunderland, MA: Sinauer Associates.)

Just how brains become wired up in regard to connections is the mystery of genetic and experiential control of behavior. Clearly both sources of knowledge are incorporated into every brain.

The above image shows the complexity of interconnections. In the above right insert a dendrite crosses an axon without connecting with it. In the lower right a synapse is formed between two neurons. Why in one case but not another?

(This complexity explains why our brains and peripheral sensory neurons need a large amount of fat in development and throughout life -- myelinated connections among neurons are fatty glial cells wrapped around axons. This "insulation" allows much more rapid transmission of information than unmyelinated cells found in invertebrates and young primates.)

2 x Larger prefrontal cortex (PFC) than expected for primate our body size

Recall the sorry story of Phineas Gage? (also info here.)There's another such case in the video on the brain shown in class. These cases suggest the significance of the PFC in planning, risk taking, social interaction, and emotion. Even monkeys trained to order their responses (first, second,..) show brain activity in the lateral PFC regardless of what objects or responses are involved.

Humans have a larger temporal lobe than expected.

"The human temporal lobe is larger in terms of overall volume, surface areas, and white matter volume, than predicted for an ape with a human-sized brain. (Rilling & Seligman, 2002, p.527"

The temporal lobes are concerned with memory. Recall another famous case, that of H.M who had both medial temporal lobes (MTL) removed in an effort to treat his disabling epilepsy. After surgery, HM could no longer form new episodic memory and his memories of events before surgery were very spotty for the 10 years prior to the surgery. He shows some ability to learn implicitly but at a slow rate.

The greatest difference between humans and apes is in the volume of the human temporal lobe white matter. This suggests that greater connections temporal lobes with cortex may be part of the evolution of language and other relatively unique human capacities based on memory.

Rilling, J. K., & Seligman, R. A. (2002). A quantitative morphometric comparative analysis of the the primate temporal lobe. Journal of Human Evolution, 42, 505-533.

Greater lateralization of functions

>distance between hemispheres may enable more independent functioning
significant communication between hemispheres
the corpus callosum, and anterior and posterior commissures enable info to rapidly pass from one side to another. Yet as brain size increased, corpus callosum connections did not keep up, fostering independent functioning.
Some sub-cortical inter communication
Dominant left hemisphere/right hand

Left hemisphere

Typically larger

Syntactic language, lexicon, generative phonology

Narrative, hypothesis formation, imaginative

Gazzaniga's 'interpreter'-

Responsible for self and consciousness?

Right hemisphere

 

Lexicon, little generativity , but some grammar and can stand-in if LH is damaged, even removed.

Veridical

 

 

(even the bilatral temporal lobes have somewhat lateralized functions)

Lateralization enables specialization and relatively independent modules.

Specific cognitive specializations

Human Language -- a unique communication tool
Social cognition (incl theory of mind)
 

Costs and benefits of increasingly large brain

Benefits debated but:

Larger size enables richer network of connections

Greater independence of modules

More memory

All add up to greater cognitive power

Costs

Our brain uses 20% to 40% of calories – adults and kids

Larger skull entails greater birth risks for mom & neonate

(limits on cervical opening size lead to greater premature births in humans, greater dependence on mom, and other social developments characteristic of humans (male investment, etc.) Our large head also stresses neck joints!

Greater cooling required to maintain temperature

Thiink of your brain as your car engine-- it needs fuel, exhaust, and oxygen delivered to the neurons on demand. Shortages cause delays, blackouts, and cell death in a short time. Glial cells play a role in mediating between blood supply that carries these materials and the neurons.( Einstein's brain is said to be different in its larger number of glial cells.)

Requires reliable large supply of fats and other nutrients to build the brain pre and post-natally.

See "news" for discussions of diet changes and effects phylogenetically and ontogenetically.

Requires waste removal as well.

The Circulatory system is critical for all of the above.

"Mirror neurons" and social learning

(see video online if not in class)

"A category of stimuli of great importance for primates, humans in particular, is that formed by actions done by other individuals. If we want to survive, we must understand the actions of others. Furthermore, without action understanding, social organization is impossible. In the case of humans, there is another faculty that depends on the observation of others' actions: imitation learning. Unlike most species, we are able to learn by imitation, and this faculty is at the basis of human culture. In this review we present data on a neurophysiological mechanismthe mirror-neuron mechanismthat appears to play a fundamental role in both action understanding and imitation. We describe first the functional properties of mirror neurons in monkeys. We review next the characteristics of the mirror-neuron system in humans. We stress, in particular, those properties specific to the human mirror-neuron system that might explain the human capacity to learn by imitation. We conclude by discussing the relationship between the mirror-neuron system and language."
Annual Review of Neuroscience
Vol. 27: 169-192 (Volume publication date July 2004)

THE MIRROR-NEURON SYSTEM
Giacomo Rizzolatti1 and -Laila Craighero2-
1

 

Genes and brains

Despite much recent effort, little for certain is known about how human brains evolved. One recent research finding is that hundreds, maybe thousands of genes mutated during the evolution of the human brain. Each change appears to make a small, yet distinct biochemical difference in the human brain.

Not surprisingly, genes relating to smell, hearing, speech, and diet seem to be most different between chimp and humans.

Primate brain development

See old notes.

Senses and the brain

Much of the brain is involved with sensory and/or motor processes. An old idea that I think has some merit, is that the neocortex and especially the PFC evolved to "disconnect" input sensory information from output motor responses, allowing more advanced conditional control ("volition") of animal responses. Limber's puppet metaphor for motor control is based on the logic of controlling movements with lots of degrees of freedom (e.g. talking vs. walking) and old studies showing variation in brain space devoted to body parts required to make such movements.

The "mirror" neurons (above) are essentially motor control neurons that are activated by sensory inputs such that observations of specific behaviors stimulate performance of those behaviors on the part of the observer -- faciliating imitation and social learning.

I call this a "social priming" of specific responses. We will discuss this later.

Primate sensory processes

See notes.

summary on human brain evolution

bipedal (3.5mya)>larger brain in stages (2+mya)>lateralization & language (1 mya?)>full language (100K ya)

All of these stages involved other developments after bipedalism -- teeth, digestive and circulatory, birth accommodations, social structure ....??? As S. J. Gould (1977) said, human evolution was a synergistic process....