2019-04-20_New_Scientist

20 April 2019 | NewScientist | 35

record shows that ungulates, which include
modern species like the cow, goat, pig,
sheep, llama, camel and horse, first emerged
during this period. These families of large
herbivores are utterly critical to human
societies around the world, providing not
just a reliable source of meat, milk, hide,
wool and leather, but also means of transport.
We ride them, load them with packs and put
them to work hauling carts or ploughs, all in
the service of human development.
But the most significant group of mammals
that sprang up during this heatwave were
the primates, the group that our own species
belongs to. These early ancestors of ours,
physically similar to lemurs, emerged and
then rapidly dispersed across Asia, Europe
and North America. But it was in the unique
geology of East Africa that they took their
first unsteady steps towards humanity.

HOW WE GOT SMART

All animals evolve in response to their

natural environment, and our own species,

Homo sapiens, is no different. The emergence

of our large brains millions of years ago is

a case in point. These supercharged organs

require a lot of resources and energy, and

so would have needed a very good reason

to emerge. Their development would have

been driven by necessity, probably as a

response to complex and rapidly changing

surroundings that required high intelligence

and adaptability to survive. But what has long

puzzled palaeontologists is what made the

corridor of the East African Rift Valley such

an ideal incubator for these intelligent apes

(see “Narrow escape”, right).

The major factor that led our evolutionary

family, the hominins, to split from our

tree-swinging, primate ancestors was the

general drying out of East Africa and the

transformation of the densely forested habitat

into grassy savannah. This drying trend was

driven largely by forces beneath the ground.

As the African continental plate bulged

upwards and ripped apart to form the East

African Rift Valley, wall-like mountainous

ridges rose to line the valley. These peaks

blocked the movement of moisture-laden

clouds and kept East Africa arid.

The amount of rainfall available to our

ancestors in this dry, equatorial region

would have varied with cyclical shifts in

Earth’s tilt and orbit around the sun,

known as the Milankovitch cycles. But these

cosmic variations happen over thousands

of years – too slow to have a significant effect

over an individual lifetime. So why would

big, adaptable brains be needed to navigate

this relatively stable new status quo?

The answer that has been emerging in

recent years relies on a powerful, combined

effect of tectonic geography and cosmic cycles.

While the mountainous walls of the rift collect

rainfall on their flanks, the valley bottom is

hot and dry. That means the many lakes strung

along the rift floor are highly sensitive to the

delicate balance between precipitation and

evaporation, and their water levels fluctuate

significantly with the Milankovitch cycles.

During particularly unstable climatic periods,

these bodies of water rapidly flicker in and

out of existence, like the light of a dying bulb.

This changing availability of water, and

therefore vegetation, animal life and food,

is thought to be what favoured the evolution

of humanity’s versatile behaviour.

The fossil evidence backs up this

hypothesis. The three most recent periods

of extreme climate variability in this

region occurred around 2.6, 1.8 and 1 million

years ago, and are each associated with

the emergence of new hominin species in

East Africa and increases in brain size. And

once the unstable, fluctuating environment

had started encouraging the growth of

intelligence, it rapidly led to other big gains,

including increasingly complex social

interaction, language and tool use. It was

in these specialised surroundings that our

own species eventually emerged from other

hominins around 300,000 years ago. >

SOURCE: ORIGINS: HOW THE EARTH MADE US

Narrow escape

The unusual topography of the East African Rift

Valley, and the sensitivity of its lakes to climate

changes, created a challenging environment that

forced our ancestors to get smart or perish

Lake

Victoria

Lake

Malawi

Lake

Tanganyika

Gulf of Aden

Small lakes Rift valley

ANOTHER PLACE, 1997,

CAST IRON, 100 ELEMENTS,

189 X 53 X 29 CM,

INSTALLATION VIEW,

SOLA STRAND, STAVANGER,

NORWAY, 1998,

PHOTOGRAPH BY DAG

MIRESTRAND © THE ARTIST