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42Evolution

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All in the head; what gives us our oddly-shaped brain?

A selection of animals with very different brains.  Clockwise from top left; Barn own (Tyto alba), a human (Homo sapiens), a Commissarisi’s Long tongued bat (Glossophaga commissarisi), and a red squirrel (Scirius vulgaris). 
(All images: Wikimedia commons)

A selection of animals with very different brains. Clockwise from top left; Barn own (Tyto alba), a human (Homo sapiens), a Commissarisi’s Long tongued bat (Glossophaga commissarisi), and a red squirrel (Scirius vulgaris). (All images: Wikimedia commons)

“I am a nectar-feeding bat: pulses of sound guiding me unerringly to the plant’s acoustic beacon.”

“I am an owl: with hearing beyond human imagination I swoop silently onto the unsuspecting shrew.” 

“I am a squirrel, as I leap across the branches unwittingly I track the same footsteps as the early primates.” 

“I am a human and I alone ask myself how it is that having inherited the brain of an ape, I can use an electron microscope whilst listening to Mozart.”


Your head contains only one mystery: your brain.

For most vertebrate animals, the bigger the body, the larger the brain: three grammes in a shrew, five kilogrammes in an elephant.  As body mass increases, the relative brain size decreases.  But there are significant exceptions where brain size dwarfs relative body size.

Brains change shape and form from one species to another.  Animals of different size, and living in very different environments can have similar brain configuration.  On the other hand, similarly sized animals playing alternative roles within the same environment have brains that seem to be very different.  This tells us that something other than body size is affecting their brain development.

This tiny nematode ‘worm’, Caenorhabditis elegans, seen here under a light microscope has a total of 959 body cells, of which 302 are neurons.   This ratio of nerves to other tissues shows that whilst these tiny worms are among the ‘simplest’ of multicellular animals, they are in practical terms have high sensory awareness and responsiveness.   (Image: Wikimedia commons)

This tiny nematode ‘worm’, Caenorhabditis elegans, seen here under a light microscope has a total of 959 body cells, of which 302 are neurons.This ratio of nerves to other tissues shows that whilst these tiny worms ... moreare among the ‘simplest’ of multicellular animals, they are in practical terms have high sensory awareness and responsiveness.(Image: Wikimedia commons)

 

The role of any brain, whatever its size, is to process incoming sensory information and translate this into bodily actions.  In our brains the numbers are astronomical: our 100 billion neurons are connected by over 100 trillion synapses (the transmission points between cells).  Many other animals are more modest.  The brain and nervous system of a nematode (roundworm) rely on 302 neurons making around 5000 synapses, but their fundamental functions are much the same.

Studies of the effects of damage in the brain suggest that many of its structures are specialised for particular tasks.  For example the cerebellum, found in vertebrate brains near the junction with the spinal cord, is connected to vital operations such as breathing and other basic bodily functions.

In the hands of the investigator, this sheep’s brain shows the expanded cerebral hemispheres(the mammalian neocortex) and cerebellum.   Only mammals have a cerebral cortex, although the pallium of birds and reptiles is frequently referred to as ‘cortex’ and fulfils at least some similar functions.     (Image: Wikimedia commons)

In the hands of the investigator, this sheep’s brain shows the expanded cerebral hemispheres(the mammalian neocortex) and cerebellum. Only mammals have a cerebral cortex, although the pallium of birds and reptiles is ... morefrequently referred to as ‘cortex’ and fulfils at least some similar functions. (Image: Wikimedia commons)

But this is not the end of the story.  Patients with cerebellar damage also struggle with learning and memory.  This shows us that higher order mental tasks, even if associated with certain regions, can only operate using the whole brain.

Obviously human brains show many similarities to those of other species.  In particular, our brain is similar to that of other primates, all of which have a characteristically expanded ‘neocortex’.  In this regard, however, it is exceptional.  There is little doubt that in comparison to our body mass, our brain size is disproportionately large.  Nevertheless it is not clear what proportion of this is due to our neocortex.

So what’s going on? If all brains are in practice an integrated, single organ fulfilling an essentially similar role, why are they so variable?

What do brains have in common?

Vertebrate brains arise from the head-most (anterior) portion of the neural tube in the embryo.  As this tube grows it folds and changes shape, forming adjacent compartments which then give rise to specific brain structures with specialised functions.

Diagram of the various parts of the brain in a six-week old human embryo.   All vertebrate brains develop from a straight ‘neural tube’, which folds over on itself to produce different brain structures.  Within this single organ, these localised structures have some specialised activity.   •  Hindbrain (Rhombencephalon) structures maintain basic functions such as the rhythmical movements driving respiration, circulation and digestion.   • The midbrain (Mesencephalon) controls posture, walking, and involuntary body movements which operate our hearing and vision.  These together comprise the ‘brainstem’.   • The forebrain (Diencephalon and Telencephalon) handles attention, sensory and spatial information processing, learning, memory, reasoning, problem solving, voluntary movements and language.   (Image: Wikimedia commons)

Diagram of the various parts of the brain in a six-week old human embryo. All vertebrate brains develop from a straight ‘neural tube’, which folds over on itself to produce different brain structures. Within this si... morengle organ, these localised structures have some specialised activity. • Hindbrain (Rhombencephalon) structures maintain basic functions such as the rhythmical movements driving respiration, circulation and digestion. • The midbrain (Mesencephalon) controls posture, walking, and involuntary body movements which operate our hearing and vision. These together comprise the ‘brainstem’. • The forebrain (Diencephalon and Telencephalon) handles attention, sensory and spatial information processing, learning, memory, reasoning, problem solving, voluntary movements and language. (Image: Wikimedia commons)

 

Brains develop as a mosaic of interdependent ‘modules’ that are scaled according to the animal’s information processing needs.  The relative size of each region compared with the overall brain size gives us what is known as the ‘cerebrotype’.  Comparing these brain types shows us that animals performing similar tasks have similar brain organisation.

Although animals have many senses, typically they rely on one or two of these for most of the information that allows them to function in their habitat and pursue their particular ecological role.  This makes their brain shape sufficiently predictable that in principle we can infer an animal’s ecological role simply from its cerebrotype.

 

Left to right; brains of a North island Brown kiwi (Apteryx mantelli), a barn owl (Tyto alba) and a domestic pigeon (Columba livia), all seen from the back.   All three have a similarly sized and shaped cerebellum at the base of the brain.   Note however, that in the kiwi, the enlarged hemispheres of the forebrain have grown over and completely hide the underlying midbrain.  The ‘Wulst’ is a region in the bird’s brain specialised for processing complex information.  In the owl brain this area is greatly expanded.   V = ‘vallecula’, a fold.  Scale bars = 0.5 cm. (Image: Wikimedia commons)

Left to right; brains of a North island Brown kiwi (Apteryx mantelli), a barn owl (Tyto alba) and a domestic pigeon (Columba livia), all seen from the back. All three have a similarly sized and shaped cerebellum at the ... morebase of the brain. Note however, that in the kiwi, the enlarged hemispheres of the forebrain have grown over and completely hide the underlying midbrain. The ‘Wulst’ is a region in the bird’s brain specialised for processing complex information. In the owl brain this area is greatly expanded. V = ‘vallecula’, a fold. Scale bars = 0.5 cm. (Image: Wikimedia commons)

What predicts a brain’s development?

The sensory context of animals varies, so a bat may hunt a moth and a dolphin will grapple with a squid, but each fulfils their ecological role using a different set of priorities and behavioural responses.

In each case, the brain must be attuned to what is required for that animal’s survival.  So if unrelated animals have similar ecological roles, we should not be surprised to find that they have converged to possess similar sorts of brains.

The North Island brown kiwi (Apteryx mantelliii) from New Zealand has adopted a nocturnal ground-foraging ecological role.   These birds have expanded brain regions for processing touch and smell.  This brain structure matches their ecological requirements; they have touch sensitive whiskers, and sensitive olfactory (scent detecting) organs at the tip of their beaks.   The kiwi’s brain, like its sensory perceptions, are similar to those of nocturnal ground foraging mammals which like them hunt for insects in leaf litter of the forest floor.     (Image: Wikimedia commons)

The North Island brown kiwi (Apteryx mantelliii) from New Zealand has adopted a nocturnal ground-foraging ecological role. These birds have expanded brain regions for processing touch and smell. This brain structure mat... moreches their ecological requirements; they have touch sensitive whiskers, and sensitive olfactory (scent detecting) organs at the tip of their beaks. The kiwi’s brain, like its sensory perceptions, are similar to those of nocturnal ground foraging mammals which like them hunt for insects in leaf litter of the forest floor. (Image: Wikimedia commons)

As a species adapts to its ecological role its brain changes the size of its ‘mosaic’ components according to their use.  Regions processing high priority information are enlarged, whilst areas of lesser importance are reduced.

For example the kiwi is unusual amongst birds in that not only is it flightless, but it relies on touch and scent to forage.  In ecological terms, it acts more like a rodent.  In evolving to specialise for this ecological role, these birds show enlargement of brain regions for processing touch and smell.  Correspondingly these same regions are expanded in the brains of rats.

Our brain is a similar shape to that of other primates that, like us, rely heavily on visual signals.  Dolphins and bats both use sonar (echolocation) to ‘see’: these animals have a similar expansion in the brain regions responsible for acoustic signal processing and vocal motor control.

Different parts of the common brain components grow and develop at different rates in different animals, resulting in often very different shapes.   (Image: Wikimedia Commons)

Different parts of the common brain components grow and develop at different rates in different animals, resulting in often very different shapes.
(Image: Wikimedia Commons)

What does this tell us about how brains evolve?

The type and availability of food in an environment are factors that drive the ecology of any animal.  The information an animal needs to process in order to succeed serves to drive jointly the evolution of both its brain and sensory apparatus.

For instance, the convergence found in echolocating bat and dolphin brains is also evident in their hearing apparatus.  A key component of our hearing anatomy is a tiny spiral-shaped structure found in our inner ears, known as the cochlea.  The so-called ‘outer hair cells’ of this organ allows us to hear fine details, particularly in sounds of certain frequencies.

Notably, in contrast to other mammals, echolocating whales and bats have shorter and stiffer cochlear outer hair cells.  These hairs contain a protein encoded by the Prestin gene, sometimes known as the ‘hearing gene’.  Identical changes in the amino acid sequence of this protein, making these hair cells more responsive to ultrasounds, have evolved independently in bats and dolphins.

 

Star nosed moles (Condylura cristata) have a mobile set of nasal ‘rays’ which act like fingers, and perceive details of the shape and texture of small items of prey and other objects.   (Image ©Kenneth Catania)

Star nosed moles (Condylura cristata) have a mobile set of nasal ‘rays’ which act like fingers, and perceive details of the shape and texture of small items of prey and other objects. (Image ©Kenneth Catania)... more

Animals use whichever sensory ‘channel’ provides the most effective information for their survival.  Star-nosed moles, for instance, spend much of their foraging time underground.  In the absence of light they have specialised to ‘see’ using touch and have evolved a nose which functions as a tactile ‘eye.

The sophistication of vertebrate senses is mirrored in their multi-lobate brains.  All things being equal, selection for faster information processing would cause an evolutionary expansion of the relevant brain regions.  Even so, selection may not favour an overall increase in brain size, let alone a larger skull or body size increase to accommodate it.  Instead as a brain module expands, the outer layer is folded so developing distinctive fissures, known as ‘gyri’.

Mammal brains are unusual in that they generally have a well-developed ‘neocortex’.  This brain region is an area of the cerebral cortex that during embryonic development derives from the forebrain.  It comprises a thin layer of ‘grey matter’ surrounding the deeper ‘white matter’, each layer of which possesses distinctive types of neuron.  Cumulatively they are involved with higher-order processing such as seeing and hearing as well as producing fine motor movements.

The grey matter of the mammalian neocortex has a 6-layered structure, housing different types and sizes of neuron.  The two views in this drawing, taken from Gray’s Anatomy, show up in thin brain sections treated with different types of histochemical stain.  The grey matter layer lies over ‘white matter’, comprised of a mass of myelinated neurons (nerves whose axons are insulated by a sheath of Schwann cells).  These connect disparate brain regions and relay signals around the whole neural network.   (Image: Wikimedia commons)

The grey matter of the mammalian neocortex has a 6-layered structure, housing different types and sizes of neuron. The two views in this drawing, taken from Gray’s Anatomy, show up in thin brain sections treated with ... moredifferent types of histochemical stain. The grey matter layer lies over ‘white matter’, comprised of a mass of myelinated neurons (nerves whose axons are insulated by a sheath of Schwann cells). These connect disparate brain regions and relay signals around the whole neural network. (Image: Wikimedia commons)

Some researchers have suggested that our primate cerebrotype is specialised for social communication.  Certainly primates with a high level of social bonding tend to have a conspicuously expanded neocortex.  Moreover human brains have many more folds and fissures than even our closest relative, the chimpanzees.

Our neocortex also includes areas required for speech and language, these being amongst our most distinctive features.

Chimpanzees maintain social bonds by devoting a large proportion of their time to grooming each other.  One idea is that our ancestors developed an early form of speech and language as a means of socially ‘grooming’ one another using sounds.  This may have allowed them to stay connected with a much larger social group, just as our various communication abilities allow us today to have potentially a huge network of friends and associates.

Thomas Huxley’s drawings of a human and chimpanzee brain scaled to the same size, from his book, Science and Education: Essays  (1904).   In practice our brains are larger than those of our closest genetic relatives, the chimpanzees.  We also have a greater number of interconnections per neuron.   (Image: Wikimedia commons)

Thomas Huxley’s drawings of a human and chimpanzee brain scaled to the same size, from his book, Science and Education: Essays (1904). In practice our brains are larger than those of our closest genetic relatives, the... more chimpanzees. We also have a greater number of interconnections per neuron. (Image: Wikimedia commons)

 

Although our brains are physically larger than those of chimpanzees, the significance of our neocortex size is a matter of considerable debate.  Nevertheless the fact remains that our brains are different.  Thus we possess greater numbers of more interconnected brain neurons than other primates.

This indicates that we process information more rapidly.  The features which contribute to our uniqueness, such as speech and creativity, evidently require greater mental processing.

Summary

  • Brains are specialised information processing organs.
  • Their overall shape (the ‘cerebrotype’) reflects the animal’s need for information in its specialised ecological role.
  • Applying this idea to ourselves suggests that our brains may be specialised for communication and creative thinking.
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