For millennia, we humans have pondered the question of where we might have come from. The Abrahamic religions proposed that we were created six days after the world by God's will. In ancient Chinese mythology, we originated from mites on the corpse of Pangu the demigod. The Norse believed that humans developed from the sweat in the armpit of a giant named YmirIn Norse mythology, Ymir was one of the two first-ever beings to exist, alongside a cow Auoumbla. Ymir was both male and female. They were killed by their sons and their body was transformed into the world..

One thing all these stories had in common was their conviction that life developed on Earth. But now this age-old belief is being challenged by new scientific evidence that the building blocks of life did not begin here at all.

Physicist Brian Cox is the latest prominent scientist to suggest the first microbes were carried from Mars to Earth on an asteroid, in a process known as "panspermiaThe hypothesis that life exists everywhere in the universe, carried on space debris, asteroids, comets and even spacecraft. While it is widely agreed that organic molecules are distributed widely, scientists disagree bitterly over whether or not single-celled organisms could survive in space.".

Organic moleculesCompounds that contain carbon-hydrogen bonds and are essential for life. exist more or less everywhere in the universe. But we still do not know how some of these fused together to form single-celled organisms.

While it was once believed that early life developed in the deep ocean, scientists increasingly believe that too much water, which dissolves many organic compounds, would have been harmful to the first microbes.

They would have benefited most from shallow waters that frequently dried up, giving them a cycle of water, light and heat, all essential for life.

The early Earth was entirely covered in vast, deep oceans that would likely have been deadly for early microbial life. But Mars had just the right amount of water for life to emerge.

The earliest evidence we have of life on Earth dates back around 3.7 billion years. It was around this time that the climate on Mars changed and it became an arid environment with no moving water.

Meanwhile, the Earth was experiencing tectonic activityThe movement of tectonic plates across the surface of the earth. It is responsible for volcanoes and earthquakes, and it causes continents to form and break apart.. The first continental platesOne of the two kinds of tectonic plates, alongside oceanic plates. They form the basis of landmasses. began to form, allowing shallow waters and then land to develop.

So just as Mars became inhospitable, the Earth was turning into the best neighbourhood in the solar system.

When Mars became a desert, most of the microbes it had fostered would have died. But if a few of these more complex single-celled organisms were able to hitch a ride on an asteroid, they could have made it to Earth, where conditions were now perfect for their evolution into the life we recognise today.

For many, this is the most exciting thing about science: big epistemological shifts that change our whole way of looking at the world.

Until recently, everyone agreed that early life formed in deep-sea vents spewing out minerals and nutrients. Now, that has all been overturned.

The same thing is happening in evolutionary biology. Since the 19th Century, all scientists have agreed that life evolves gradually and continuously. 

But now some scientists are challenging this. They think evolution happens quickly in short bursts, followed by long periods in which species do not change much at all.

That is science as it should be: all beliefs can be called into question, no matter how long they have been accepted as truth.

Yes: The chances of life emerging in two different places are astronomical. If we do discover life of any kind on Mars, it will be compelling evidence that all life on Earth started on the Red Planet and made its way over.

No: Single-celled life would have struggled to survive a trip on an asteroid. And there is no reason why the Earth could not have provided the right conditions for life to develop once land started to emerge.

Or... There is still much we do not know about how early cells worked. We do not know if they were anything like the cells we have today. More study is needed before we can be sure where life emerged.