Mammals squared

In a previous post, we summed up what makes mammals special: not a single trait, but a general trend.

[T]he over-arching attribute manifested by the origin of the mammals is increasing homeostatic ability: the maintenance of a constant internal environment in the face of a fluctuating external environment, by means of high-energy regulatory processes.(Kemp p. 18)

The same trend continues, even further, in hominin evolution. Humans are mammals squared.

Some parallels below, for particular traits.

	Mammals (vs. reptiles)	Hominins (vs. apes)
Energy budget	High basal metabolic rate (BMR) allows sustained aerobic activity	Typical mammal BMR, but higher total metabolism. External energy (fire) for higher total energy.
Rate of food collection	High rate of food collection	Higher rate of food collection. Ecological engineering.
Mechanical processing of food	Complex dentition	Tools to chop, grind, mash food, allowing reduced dentition
Energy for processing food	Higher metabolism means more energy for digestion	Fire means cooking
Energetic investment in young	Mothers do gestation, lactation, infant care	Others give mothers & young extra food and support
Body covering for thermoregulation, adornment	Fur	Clothing
Posture	Limbs under body, sustained high speeds	Bipedalism, endurance running
Brain size	Large brains	Ginormous brains
Behavior	Learning, social learning, play	Shared intentions, cumulative culture, language, imagined worlds
Social organiztion	(Some) complex social organization, recognize individuals, others’ social relationships	(Some) enormous organized social groups,  roles, norms, social scripts
Time scale (years)	100 millions	Millions

Interesting parallels. But the last line of the chart show a striking difference. Mammals developed their special way of life over hundreds of millions of years, hominins over just millions. The triumph of mammals was a triumph of biological evolution that required the slow coordinated evolution of a complex of traits. The triumph of hominins over millions of years was (largely) a triumph of cultural evolution: some somatic adaptation, and a lot more extra somatic.

Ice Age gear shift

808 – 765 thousand years ago

Around today’s date, there was a shift in the nature of glacial cycles.

But let’s back up a bit. Earth’s climate took a turn toward cool in the transition from Eocene to Oligocene, 35 million years ago (although with some warming in the Miocene). It was probably back then that much of Antarctica started being covered by ice. The establishment of open water all the way around Antarctica may have helped isolate and freeze the continent. And declining carbon dioxide levels, partly a result of weathering of rocks in the Himalayas, probably also made a difference. But it was back at the beginning of the Pleistocene, now dated to 2.5 million years ago, that the current Ice Age truly began, with glaciers covering large parts of northern North America and northern Europe.

Current Ice Age? Glaciers covering large parts of northern North America and northern Europe? This isn’t what the climate has been like for the past 12,0000 years. Within the current long Ice Age there have been long glacial periods and shorter interglacials, and we’re currently in an interglacial. Our own activities may have done something to prolong the interglacial, and stave off the return of the ice; more on this another day.

Three astronomical cycles govern the rhythm of glacial and interglacial. There’s a 100,000 year cycle as Earth’s orbit changes from somewhat more elliptical to somewhat more circular. There’s a 40,000 year cycle as Earth’s axis shifts from slightly more tilted (24.5 degrees off vertical) to slightly less (22.1 degrees); it’s currently tilted at 23.5 degrees. And there’s a 21,000 year cycle generated as the Earth precesses – wobbles like a top. Right now the North Pole is pointed at Polaris, and the Sun very recently started rising in the constellation Aquarius at the Spring equinox: hence the Age of Aquarius.

(An even longer 400,000 year cycle might have been involved earlier in human evolution, in establishing intervals in which “amplifier lakes” flashed in and out of existence in the African rift valley. More here.)

Between 2.5 million and 800,000 years ago, the glacial/interglacial alternation was dominated by the 40,000 year cycle. But beginning about 800,000 years, there has been a gear shift: the 100,000 year cycle has been dominant and swings in climate have been more extreme. (In Africa however the 21,000 year cycle is more important for alternations between rainy and dry. Africa is in a dry state now.)

One of the startling findings to come out of the last few decades of work on ice cores from Greenland and Antarctica is that not only have there have been huge long-term changes in climate, but there have also been extreme short term shifts, probably connected with changes in ocean currents. There have been a number of occasions over the last hundreds of thousands of years during which average temperatures shifted by 10-20 degrees Fahrenheit (5-10 degrees Celsius) for a millennium, or even for a century or less! (During the last 10,000 years, however, the climate has been unusually stable.)

This is bound to have had strong effects on human beings. Two anthropologists, Robert Boyd and Peter Richerson, who work on mathematical models of cultural evolution, have a general theory of how this pattern of oscillations might have affected human evolution. They argue that human adaptation takes place on multiple time scales. On very long time scales, human beings adapt to changes in the environment genetically. On very short time scale, human beings adapt to change through individual learning. But when change happens on intermediate time scales, adaptation takes place through social learning. With changes on intermediate time scales, your ancestors may not have had enough time to adapt genetically to the current climate, but things may have been stable for long enough that your culture and the wisdom of the elders have a lot to teach you about how to cope. One of the really distinctive features of human beings, maybe even The Secret of Our Success is that we are, more than any other creature, a cultural animal, with high-fidelity cultural transmission; this trait may have been shaped by the nature of climate change especially over the last 800,000 years.