34 | NewScientist | 20 April 2019raising the temperature on the surface.
Then, as now, huge deposits of methane lay
on the sea floor, a by-product of decaying
organic matter. Under the extreme pressures
and low temperatures found at such depths,
the gas became trapped within crystals of ice,
safely locked away so long as the ice didn’t
melt. In planetary terms, it was a barrel of
gunpowder waiting for a match.
The fateful spark is thought to have
been a cluster of volcanic eruptions that
peppered the atmosphere with enough
carbon dioxide to cause an initial temperature
rise. This, in turn, melted ice on the sea floor,
causing methane to bubble up through the
water and into the atmosphere, leading to
a further temperature rise that melted yet
more methane.
The sweltering climate resulted in a burst
of evolutionary diversification. The fossilHow Earth
shaped us
Lewis Dartnell charts the planetary forces
that made humanity what it is, from our big
brains to the first civilisations
HOW WE EMERGED
Some 55.5 million years ago, Earth’s
thermostat did something unexpected. In the
space of 100,000 years, barely the blink of an
eye on geological timescales, the temperature
of the planet jerked up by between 5°C and 8°C,
hovered for a bit, and then came back down.
This brief planetary fever was hugely
disruptive to life on Earth, driving the rapid
evolution and divergence of whole new
orders of animals, including our own.
The principal culprit is thought to have
been methane. As a powerful greenhouse gas,
its presence in air makes our atmosphere trap
more of the sun’s heat than it usually would,H
UMANITY today is actively reshaping
the planet. Our appetite for natural
resources and large-scale industrial
activity is eradicating species, warming the
oceans and disrupting the global climate on
an unprecedented scale. So profound is our
impact that some have called for the times we
live in to be declared a new geological period:
the Anthropocene, the age of humanity.
But this ability to shape our environment
on such a scale is a recent phenomenon. For
most of our history, it is our environment
that has shaped us. The physical features
of the planet we live on enabled our species
to arise, nurtured our remarkable brains,
facilitated our spread across the planet and
even encouraged the birth of the first cities.
This is the remarkable story of the way Earth
has moulded humanity, and how we have
turned the tables to shape the world.