William Reville: Is there life beyond Earth?

Based on current understanding of how our planet formed, the answer is almost inevitably yes

Photograph: Thinkstock

Photograph: Thinkstock

 

Two of the largest questions confronting science are: (a) how did life begin on Earth, and (b) does life exist elsewhere in the universe? Although we think we know the answer to the first question in principle – life spontaneously arose through the self-organisation of simple chemical building blocks – we remain a long way from a detailed understanding of the process. But, if our understanding of the origin of life on Earth is correct, then it seems inevitable that the vast universe beyond Earth also teems with life. Current research in this area was summarised by Jeffrey Kluger in Time in February.

In a landmark 1953 experiment, Stanley Miller and Harold Urey of the University of Chicago recreated the atmosphere of early Earth (a mixture of water vapour, ammonia, methane and hydrogen) and heated and irradiated it with high-voltage electric sparks (representing lightning). This treatment produced many of the 20 amino acids that are the building blocks of proteins, as well as organic acids and sugars found in living cells.

Analogous but more complex experiments are ongoing at the Nasa Ames Research Centre, where Scott Sandford and others simulate interstellar space (dilute gases and dust at a temperature of minus 233 degrees) and irradiate it with energetic radiation of the kind found in interstellar space. Thousands of chemical products are formed as a result, including amino acids, the purine and pyrimidine building blocks of the genetic materials DNA and RNA, and many sugars found in living cells.

We know that organic molecules vital for life are formed extraterrestrially, because amino acids are routinely found in meteorites that fall to Earth. And water, which is essential to life, is the most common compound in the universe. The Nasa simulation experiments show that when Earth formed, many of the basic building blocks of life were likely to have been present from the start. This is equally likely wherever planets are formed in the universe, since the Nasa simulations employ universal astrophysical conditions (Ruth Marlaire and Steve Koppes, University of Chicago News).

If life exists elsewhere in the universe, it presumably lives on a planet, but as recently as 20 years ago we knew of no other planets outside our solar system. However, the Kepler space telescope, launched in 2009, has detected almost 5,000 candidate-planets, of which 1,039 have been confirmed, looking only into one tiny patch of our Milky Way galaxy. To support life such as ours, a planet must also orbit its sun in the so-called Goldilocks zone where water can exist in the liquid state, and should be about the same size as Earth, have a rocky surface and be capable of holding on to an atmosphere.

The Nasa simulations also produce another class of molecules called amphiphiles: long hydrocarbon chains with a water-loving and a water-hating end. In an aqueous environment, these amphiphile molecules self-assemble into membranes with the ends that “like” water on the outside facing water, and the ends that “hate” water buried in the interior of the membrane, away from water. This is the basic structure of the membrane that surrounds every living cell today.

Before life began, there must have been a phase of chemical evolution where membrane-bounded nonliving “cells” segregated their contents from the outside environment, allowing internal chemical composition to diverge from conditions outside. These cells could “eat”, grow and divide. Eventually an information storage system, probably based on RNA, arose within the cells, allowing them to remember what they were doing and to pass on improvements to their descendants. This last complex step was the origin of life; biological evolution could now begin.

If we are right about how life spontaneously started on Earth, it seems chemically and mathematically inevitable that it also arose in many locations throughout the universe. As Sandford said, “The universe is hardwired to be an organic chemist. It’s not a very clean or tidy one, but it has very large beakers and plenty of time”. It seems the universe cannot avoid producing life.

William Reville is an emeritus professor of biochemistry at UCC http://understandingscience.ucc.ie

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