The
idea of discovering a new form of life has not only excited astronomers
and astrobiologists for decades, but also the wider public. The notion
that we are the only example of a successful life form in the galaxy
has, for many, seemed like an unlikely statistic, as we discover more
and more habitable planetary bodies and hear yet more evidence of life’s
ability to survive in extreme conditions. A new essay, published May 8
in the online, open-access journal PLoS Biology,
examines what really constitutes ‘life’ and the probability of
discovering new life forms. Accompanying the article is an interview
with the author in the latest edition of the PLoS Biology Podcast.
Professor
Gerald Joyce, from The Scripps Research Institute in La Jolla,
California, discusses in the essay the basic requirements for a life
form to exist. He says, “Life self-reproduces, transmits heritable
information to its progeny, and undergoes Darwinian evolution based on
natural selection.”
He
refers to this heritable information as ‘bits’ (for life as we know it,
this includes the four bases of DNA), and explains that although
Darwinian evolution results in new combinations of these bits, this does
not define a new or alien form of life. Indeed, to date no truly new
life form has been discovered—either in extreme environments on Earth or
on other planets—that contains new bits, despite evidence suggesting
life on meteorites recovered in Antarctica, or on any of the so-called
‘habitable’ planets discovered in our galaxy.
How
could a truly new life form arise? Joyce explains that an organism
could either arise directly through chemistry, or spin off from existing
biology. For the former, a life form would self-organize “into a
bit-generating system.” It’s thought that this is how life originated on
Earth; from a primordial soup of chemicals in an aqueous environment
that generated self-replicating molecules, which then mutated and
evolved. Joyce argues, “A life form that arises directly from bit-free
chemistry would be considered ‘new’ from the outset, while one that
derives from a biological cell would have a long way to go before
reaching the threshold.”
It
is in these differences between chemical or biological initiation—that
is to say, whether the life form has developed from an existing life or
seemingly independently—that confusion and misinformation occurs
surrounding the probability of a new life being discovered or created.
Given that we only know of one life form—our own—we can’t meaningfully
estimate the probability of new life arising, either on Earth or
elsewhere.
“I
think humans are lonely and long for another form of life in the
universe,” says Joyce, “preferably one that is intelligent and
benevolent. But wishing upon a star does not make it so. We must either
discover alternative life or construct it in the laboratory. Someday it
may be discovered by a Columbus who travels to a distant world or, more
likely in my opinion, invented by a Geppetto who toils at the
workbench.”
In the accompanying PLoS Biology
Podcast, Joyce discusses in more depth the ability of scientists to
discover the origin of novel life forms, and whether the emerging field
of synthetic biology can actually lead to new forms of life.
Bit by Bit: The Darwinian Basis of Life
Source: Public Library of Science