2019-04-20_New_Scientist

36 | NewScientist | 20 April 2019

HOW WE TOOK OVER THE WORLD

Our ancestors didn’t remain in their cradle.
The narrow, tube-like rift valley squeezed
migrating populations out of its northern
end. During wetter, greener spells, they would
have been able to walk across the usually
barren plains of North Africa to Sinai, in what
is now north-eastern Egypt, or the Arabian
peninsula beyond. But to truly take over the
world, including by crossing the oceans that
separated continents, an entirely different
set of planetary conditions would be needed.
The first of our ancestors’ big migrations
began nearly 2 million years ago, when
Homo erectus spread across Asia, reaching
as far as China and Indonesia. There they gave
rise to at least two other hominin species – the
Neanderthals in Europe and the Denisovans in
central Asia. Anatomically modern humans
left Africa around 65,000 years ago, spreading
up into Europe and along the southern margin
of Eurasia to today’s India and South-East Asia.
During this great dispersal, Earth was in
the depths of its most recent freeze, one of at
least 40 such glaciations that have taken place
over the past 2.6 million years. Not only were
global temperatures much lower during these
periods, but the climate was also much drier.
Even beyond the reach of the advancing ice
sheets, much of the land was desolate tundra.
Such conditions would have been undeniably
brutal for palaeolithic humans living across
Eurasia, but the glaciation offered one crucial
advantage to our early ancestors. The great
ice sheets locked up vast amounts of water,
lowering global sea levels by up to 120 metres.
Large tracts of the shallow continental
shelves became dry land, and this offered flat
plains for hunting and highways for migration
(see “Frosty reception”, above). Early humans
in Asia were able to simply walk across the
Sunda land bridge to populate current-day
Malaysia, as well as Sumatra, Java and other
parts of Indonesia, while the Sahul land bridge
offered easy access between what is now
New Guinea and Australia. But perhaps most
significantly for the human story, a wide
corridor of land linking eastern Siberia and
Alaska – the Bering land bridge – also emerged.
This gave our ancestors a route of entry from
Eurasia into the otherwise unreachable
continent of North America, where they soon
worked their way down to cross the isthmus
of Panama and reach South America as well.

As they moved into Europe and central Asia,

our ancestors encountered – and interbred

with – their Neanderthal and Denisovan

cousins. But after crossing into the Americas,

humanity was walking where no hominin

species had ever trodden before. When the last

glaciation ended and the Bering land bridge

disappeared back beneath the waves, east

and west became severed. Two isolated human

populations, essentially identical in terms

of genetics and abilities, but with access to

different sets of plants and animals, began

independent experiments in putting down

roots and building cities.

HOW WE BUILT CITIES

From around 11,500 years ago, people all

over the world began abandoning their

hunter-gatherer ways and started to settle

down. There were good reasons for this

lifestyle change. The most recent glacial

period was coming to an end, marking the

first interglacial period that early humans

had experienced since migrating out of Africa.

The period of relative climatic stability that

has lasted since then proved ideal for the

emergence of agriculture and city life,

spawning the civilisations that have

irrevocably shaped present-day humanity.

In short order, we learned to tame the

natural world to provide domesticated animal

and plant species for our farms. The staple of

most of our diets became cereal crops such as

wheat, rice and maize – grass species that had

proliferated around the world as Earth cooled

and dried over the past few tens of millions of

years. And the animals we domesticated were

mostly ungulates – herbivores like the sheep,

pig, cow and horse, whose ancestors had come

to dominate these new grassy ecologies.

Settled farming allowed populations

to expand quickly, and soon people began

congregating in dense clusters that became

the first cities. But it was planetary forces

that came to dictate where these first cities

formed and the earliest civilisations began.

Take Mesopotamia. This was where

the organisation, governance and cities

of Sumerian civilisation began emerging

around 6000 years ago, in an area roughly

corresponding with the northern part of

the modern day Middle East. The Tigris

and Euphrates rivers reliably delivered

the lifeblood for irrigating agriculture,

and deposited thick alluvial soil carried

from the highlands to the north-west.

The reason so much fertile soil built up in

Mesopotamia is because it lies along what is

known geologically as a foreland basin. This

feature is the result of continental drift that

caused the Arabian peninsula to swing away

from north-east Africa. It slammed into the

southern margin of the Eurasian tectonic

plate, forming the Zagros mountains, and the

immense weight of this mountain range flexed

Frosty reception

The lowered sea levels of Earth’s frozen spells exposed more coastal land (green) that served as land bridges

for Homo sapiens to migrate around the world. Dates are oldest evidence of H. sapiens

20,000

years ago

12,500 years ago

11,000 years ago

150,000-

200,000

years ago

Migration paths of H. sapiens

60,000-

70,000

years ago

45,000-50,000 years ago

45,000

years ago

40,000 years ago

Bering land bridge

Sunda land bridge

Sahul land bridge

SOURCE: ORIGINS: HOW THE EARTH MADE US