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A graduate student summary of the public lecture
Did life on Earth come from Mars?
by Paul Davies
Winning Showcase essay by Santosh Gupta

In 1859, Charles Darwin published his famous work the Origin of Species, which gave an account of how life evolved. However, it left many questions open which today remain unanswered. Perhaps the most significant question is ‘how did life get started in the first place?’ a question that seeks to clarify the position of humankind in the universe. The answer may lead to answers for two other questions: “are we alone in the universe?” and “is life just a ‘freak accident’ of a sequence of chemical events or do we live in a bio-friendly cosmos?” If life is just a freak accident then it is very likely that we are alone, while if we live in a bio-friendly cosmos, there is a better chance that other intelligent civilizations thrive in the universe, perhaps even in our own Milky Way galaxy.

Questions about the origins of life can be divided into the categories of ‘when,’ ‘where,’ and ‘how.’ The fossil record allows us to trace the evolution of life backwards in time and determine when life originated. The origins of microbial organisms can be found in 3.5 billion years old rocks of the Pilbara region of Western Australia. These rocks, known as stromatolites, are geological formations created by microbes, and are the world’s oldest known fossils. They give investigators an idea of when life first emerged near the surface of Earth.

There are many views regarding where life originated. Darwin speculated that life originated in a warm, little pond in which chemicals would self-assemble into organisms. This ‘primordial soup’ was thought to have existed 4 billion years ago and required sunlight as an energy source. Another view is that life came to Earth from space in the form of microbes carried in cosmic impacters. A more recent view is that life began deep inside Earth, in conditions of extreme heat and chemical potency, and that the first organisms were ‘superbugs’ capable of surviving these harsh conditions. All these views are still much debated, though in recent years there has been mounting evidence (for example, from observations of thermophiles) for the idea that life started out hot and deep.

The question of how life originated is probably the most difficult. The evolution of life can be traced from the beginnings of the solar system. The solar system itself formed about 4.6 billion years ago. When the proto-earth struck another body, its layers were stripped off and it formed the Moon. Bombardment by astronomical impacters of up to 500 km across continued upon the earth for 700 million years, forming the craters evident on the terrestrial and lunar surfaces. As a result, the surface of the Earth was very hot up to 3.8 billion years ago. This leaves only about 0.3 billion years between the end of the bombardment and the beginning of life, according to the fossil record. How can such a short span separate these two dramatic episodes of the earth’s history? Scientists now believe that life could have existed during those harsh periods of bombardment. The view that life started deep inside Earth’s crust, relatively safe from the bombardment, could solve this mystery.

This idea sheds new light on the question of whether life could have existed on Mars. Since the Viking landings in 1977, there has been a wide consensus among scientists that the Martian surface is lifeless. However, there is some evidence that life may have existed on Mars in the past; liquid water existed there in the forms of oceans and lakes 3.5 billion years ago. In addition, Mars likely had enormous atmospheric pressure and an enormous greenhouse effect. It is quite possible that Mars initially had a more suitable environment for life to begin than did the Earth. However, Mars is less capable of sustaining life compared to the Earth. Its smaller size leads to a weaker gravitational field, making it less able to support a reasonably thick atmosphere. The loss of an atmosphere would lead to the evaporation of water from its surface, explaining why its ancient channels are dried up and why life cannot survive.

If life existed on Mars, it may have spread to Earth through continued bombardment of big impacters 20 km across with a speed of 30 km/s. It is speculated that the impacts would have been violent enough to send off rocks from deep inside Mars, containing living organisms, towards the Earth. The microbes themselves could survive the impact, and once out in space the cold vacuum would preserve the microbes rather than destroy them. Their metabolism may be shut down, but the microbes might survive for millions of years under these conditions.

If compelling proof is found that life on Earth did indeed originate on Mars, this could support the idea that life is resilient, able to spread between planets, and to some extent is favored by the laws of Nature. To what extent life is favored, however, would remain a mystery. Until this mystery is resolved, humankind should devote significant attention to questioning the origins of life. We may one day learn that we are not alone in the universe, that other intelligent civilizations exist on extra-terrestrial planets, and that our own existence may not have been such an accident after all.

Pacific Institute for Theoretical Physics
University of British Columbia
Hennings Building, 6224 Agricultural Road
Vancouver, BC V6T 1Z1, Canada