Humans emerged thanks to Earth colliding with cold clouds, study suggests

Earth passed through a giant interstellar cloud two million years ago which cooled the planet and may have triggered the emergence of humans, a new study suggests.

Usually Earth sits in a protective bubble called the heliosphere which is created by the Sun and keeps the Solar System cocooned from deep space radiation and galactic rays.

But scientists say there is evidence that around two million years ago, the Solar System ran into a cold interstellar cloud so dense that it pushed back the heliosphere, leaving Earth exposed to the rigours of deep space.

The timing matches a cooling period in Earth’s history which coincides with the emergence of archaic humans who may have evolved to cope with climate change.

“This paper is the first to quantitatively show there was an encounter between the Sun and something outside of the Solar System that would have affected Earth’s climate,” said Merav Opher, a professor in astronomy at Boston University.

“Stars move, and not only do they move, but they encounter drastic changes.”

Around two million years ago, the earliest known humans including Homo Habilis and Homo Erectus, who walked upright and could use tools, emerged.

Our earliest human ancestors lived alongside sabre-toothed tigers and mammoths at a time when Earth was regularly plunged into multiple ice ages.

Scientists previously believed these shifts in climate occurred because of the planet’s tilt and rotation, shifting plate tectonics, volcanic eruptions, and carbon dioxide levels in the atmosphere.

But the new study suggests that Earth’s climate can also be impacted by where the Solar System is located in the Milky Way galaxy.

The Solar System orbits around a black hole at the centre of the Milky Way, meaning it passes through different areas of interstellar space.

Researchers from Boston University looked back in time to calculate where the Sun was positioned two million years ago and also mapped the path of a string of large, dense, very cold clouds mostly made of hydrogen atoms called the Local Ribbon of Cold Clouds system.

Simulations showed that one of the clouds close to the end of that ribbon, named the Local Lynx of Cold Cloud, could have collided with the heliosphere.

If that had happened, Earth would have been fully exposed to the interstellar medium, where gas and dust mix with the leftover atomic elements of exploded stars, including iron and plutonium.

New ice age

Although the researchers say it is impossible to know the exact effect, they say it could have tipped Earth into a new ice age.

The timing matches with temperature records that indicate a cooling period and also align with geological evidence that shows increased iron and plutonium isotopes in the ocean, in Antarctic ice cores and on the Moon.

Writing in the journal Nature Astronomy, the authors said: “With the shrinkage of the heliosphere, the Earth was exposed directly to the interstellar medium.

“It has been suggested that climate changes around this time could have affected human evolution.

“The hypothesis is that the emergence of our species Homo sapiens was shaped by the need to adapt to climate change.”

Pressure from the cloud could have continually blocked out the heliosphere for a couple of hundred years to a million years, researchers believe, before the Solar System moved away again, and the heliosphere was restored.

Avi Loeb, director of Harvard University’s Institute for Theory and Computation and co-author on the paper, said: “It is exciting to discover that our passage through dense clouds a few million years ago could have exposed the Earth to a much larger flux of cosmic rays and hydrogen atoms.

“Our results open a new window into the relationship between the evolution of life on Earth and our cosmic neighbourhood.”