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Extra-terrestrial life probably like life on Earth

Charles H. Lineweaver and Aditya Chopra

Planetary Science Institute, a joint initiative of Research School of Earth Sciences and Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 0200, Australia

When it comes to studying life 'out there', a lot can be learnt from life 'down here'.

In a effort to identify features of possible extraterrestrial life, astrobiologists Dr Charles Lineweaver and PhD Student Aditya Chopra at the Planetary Science Institute, have put together a list of features that are common to all life forms on Earth and hence are the most likely features of terrestrial life that could be shared by life elsewhere in the universe.

In a time when we are starting to find habitable planets beyond our Solar System and on the verge of visting sites within our solar system such as Mars and Jupiter's moon Europa to look for life, Dr Lineweaver says "it is necessary for us to consider what signs of life we should be looking for in these places... and what better place to guide our search than what we find at home."

Some of the most fundamental features of life on Earth are that all known life forms use liquid water as the solvent and carbon as the scaffold for biochemistry. All life forms also have a similar stoichiometry of the major bioelements such as nitrogen, phosphorus and sulphur. Lineweaver and Chopra's research suggests that despite the differences and peculiarities of all the different life forms on Earth, be it a tiny cyanobacterium or a shrew mouse, they are remarkably similar in their elemental composition. The authors suggest that the main reason for this similarity is that the composition of life reflects to a large extent the composition of the environment. Since water is the most abundance solvent and carbon is the most abundant element that can form complex biomolecules, not just on Earth but also across the universe, it is more likely that any extra-terrestrial life will also be based on carbon and water, rather than being based on silicon or ammonia-based biochemistry.

Other features that the authors have identified are common to life on Earth and hence likely to be universal include the LEGO principle, homochirality, harnessing free energy from thermodynamic disequilibria and the Darwinian evolution of inheritable molecules. By examining the earliest divergences of life on Earth, the authors also predict that the earliest life forms on other worlds would be hyperthermophilic or heat-loving.

Lineweaver and Chopra argue against expecting other frequently espoused candidate features of extra-terrestrial life such as multicellularity, sexual reproduction, and human-like intelligence because they are based on subjective notions of universal fitness.

To be published in Lineweaver, C. H., & Chopra, A. (2012). What can Life on Earth Tell Us about Life in the Universe? In J. Seckbach (Ed.), Genesis - In The Beginning: Precursors of Life, Chemical Models and Early Biological Evolution (p. 804). Springer. ISBN 978-94-007-2940-7.

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