Aliens may be living among us, but we do not know it because they are microbes that do not have the standard biochemistry of Earth-dwelling organisms.

As well as the many forms of life based on DNA that are known to science, the Earth may have been home to a second creation of organisms that make up an unremarked realm of “life as we don’t know it”, according to Paul Davies, of Arizona State University, a cosmologist and theorist of extraterrestrial life.

Such “weird life” would never have been identified by scientists because the techniques they use for studying microbes are based on the familiar biological processes that drive the living things we understand, Professor Davies told the American Association for the Advancement of Science conference in Chicago.

The identification of such life on Earth could aid efforts to find life on Mars or elsewhere in the solar system. A second terrestrial creation would also indicate that life arises easily when the conditions are right, suggesting that it is common throughout the Universe.

The search for aliens should thus begin at home in a “mission to Earth”, Professor Davies said.

“When I was a student in the 1960s, almost nobody believed there was any life beyond Earth. The prevailing view was that it was a statistically improbable freak accident, a chemical quirk that happened only once in the observable Universe and we are it. In the past few decades the pendulum has swung, and it’s now quite fashionable to say the Universe is teeming with life. But the scientific facts haven’t changed that much – we’re still completely ignorant of whether life emerges pretty much automatically in Earth-like conditions, or whether it’s just a fluke.

"The belief that life is widespread is based on the idea that it’s easy to make in Earth-like conditions. There is a rather obvious way [of testing this], without even leaving our own planet. There’s no planet more Earth-like than Earth itself, so if life does form readily in those conditions shouldn’t it have formed many times over right here on our own planet?

“There’s been no systematic search for life as we don’t know it here on Earth. There’s been plenty of thought given to weird life on Mars or Europa or elsewhere, but very little thought to what weird life on Earth might be like, how it might manifest itself.”

He suggested several forms that weird life might take. Known life on Earth is based on five main elements – carbon, hydrogen, oxygen, nitrogen and phosphorous – but it is possible that organisms could use arsenic instead of phosphorous. Arsenic is poisonous because it so easily replaces phosphorous in our biochemistry.

Many of the molecules of life are also shaped in left- or right-handed fashion – the double helix of DNA is always a right-handed spiral – and this seems to have been determined randomly. A microbe that uses left-handed DNA could exist unrecognised.

Some microbes may also have a means of carrying genetic information and replicating themselves that is not based on DNA, or that has extra DNA “letters”. These microbes could exist in extreme environments such as deep underground or in hot springs, or they could even live inside other organisms, including ourselves. “They might be right in front of our noses, or even in our noses,” Professor Davies said.

“In the microbial realm some of those little organisms might have an alternative biochemistry that might represent a second or subsequent genesis event. We can imagine a series of stop-go experiments in which life was formed and annihilated again and again, and it’s entirely possible that more than one form was left. Then there’s the issue of whether it could have survived to today, and formed a sort of shadow biosphere of all things that are not like the life we know.

“We can make an educated guess as to what this alternative life might be. Almost all of the techniques that microbiologists use are customised for life as we know it. If you go out looking for A, you’re not likely to find B, so it’s no surprise that microbiologists haven’t come across microorganisms that have radically different biochemistry.”

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