Tuesday, March 02, 2010

The Cost of a Brain

Homo is the product of at least a billion years of biological evolution on planet earth. To be sure not much happened until maybe 600,000 years ago, but when multicellular eukaryotic life appeared the pace of evolution increased dramatically.
No single organism achieved global domination before Homo. A big brain delivers biological success;  beating size, strength, speed and fecundity.

Or does it? Monkeys and apes have big brains, and many of the attributes of Homo, but it is hard to claim they achieved spectacular biological success.
I suspect that language, so far unique to Homo, is the most important element in the success of Homo.
Language is the operating system of the brain, permitting thought and creativity, disseminating information, and enabling complicated social structures.
The corollary to this is that personality will be influenced by the qualities of the language learned in childhood.
A brain capable of language distinguishes humanity.

But brains are biologically very expensive. The demands of a big brain dominate the anatomy, physiology and behavior of Homo, with consequences which are often surprising.

Consider the anatomy of Homo. Intelligence needs a means to manipulate objects, to examine and to use. The human hand is a remarkable multi-purpose tool, the peripheral essential for thought and intelligence. Its fine control attests remarkable co-ordination between hand, eyes and nervous system.

Bipedalism with the spine erect is an anatomical necessity for an intelligent brain, demanding sophisticated neuromuscular control: brain and brawn mutually dependent. Bipedalism frees the fore-limb: locomotion becomes a function of the hind-limbs only.

Bipedalism imposes penalties. The hind-limbs must carry the body weight, generate power for locomotion, and cope with the loads generated. Human leg joints must be among the most stressed in nature: abnormally tall individuals suffer arthritis from an early age.

Bipedalism developed in dinosaurs and birds, but with the spine horizontal. In birds this permitted the evolution of wings; in most bipedal dinosaurs problems of balance mean head, mouth and fore-limbs must be light and small, and the tail big and heavy. In later dinosaurs better posture -  semi-erect - made this problem less severe, so more versatile fore-limbs are found.
Does this mean that dinosaurs might have evolved big brains, even speech and intelligence, but for the extinction event?
Probably not - they lacked the tight control of the internal environment necessary for advanced brain function.

Sense organs must provide precise information: stereoscopic colour vision, acute hearing, and finely discriminating touch, especially on the fingers. Scent and taste are less important, so the muzzle is reduced.
Perception is further improved by the balance of the head, high on a mobile neck - more joints severely stressed in Homo, as the prevalence of cervical osteo-arthritis testifies.

Human physiology supplies the brain, disposes of its wastes, maintains its precise temperature and chemical environments, and delivers its high energy demand.
As a rough guide, in average conditions, a whole day's human energy may be divided three ways: one part for the brain, one to maintain body temperature, and one for all the rest.
Homo needs a lot of food. This is a serious biological penalty incurred to have a brain capable of language.
The brain takes priority in human nutrition, especially during development. If malnutrition or placental insufficiency retards foetal growth, the baby is born with a near normal head, but a short and stunted body.

But Homo's biggest biological penalty is the long growth and maturation of the brain: at least 10 years infancy.
The human baby is a biological absurdity: helpless, exceptionally vulnerable and slow growing. It demands unprecedented care and commitment, from mother and father, bonded intensely and durably. Some say the survival of grand-parents beyond their reproductive years evolved because their additional support helped more grand-children reach maturity.
Social behavior is essential for human survival.

Sex is intense in Homo. It is the primary bonding between reproductive male and female. It is reinforced by powerful emotional responses, the stuff of so much human artistry.
The human female trades sex and guaranteed paternity for commitment, support and security - that is the big deal done at the dawn of man. In chimpanzees sex strengthens clan bonding, but in human evolution the bargain became more intense and more specific.

Essential to this bonding function is loss of the usual mammalian oestrous cycle, in which the female ovulates and becomes receptive to males for short periods only, in due season and time after pregnancies. Instead the human female has the menstrual cycle, in effect continual oestrous, cycle following cycle without intermission, receptive to a chosen male for most of the time.
So menstruation is an adaptation to the biological demands of a big brain!

Too frequent pregnancy is a complication of this behavior, threatening bonding and the future of children. In 'natural' human societies female fertility is suppressed after pregnancy, especially by lactation, which may go on for years.
In well fed modern societies such suppression seems to be lost, and a hard lesson must be learned. Limited numbers of intensively reared children are essential for civilization.

The importance of sex is shown by the strict regulation of sexuality in most human societies. Custom, law and religion focus powerfully on this issue, prescribing severe and sometimes savage penalties for disobedience.
Religion an adaptation promoting the survival of a big-brained species? Hmm. I'll have to think about that.

Homo is a very high maintenance organism. This is the root of the territorial imperative, the need to own resources for the kindred / cultural social unit, excluding competitors. This behavior is  seen already in chimpanzees.
War is another unforeseen consequence of the demands of a big brain.

Indeed the costs of a big brain are exacting, extensive, exigent, and often unexpected. But so far the benefits have amply justified the investment. And even a brief contemplation prompts another important conclusion:  

E biologia semper aliquid novi. 

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