Life on Earth… but not as we know it

The idea of the shadow biosphere is also controversial and is challenged by several other scientists. “I think it is very unlikely that after 300 years of microbiology we would not have detected such organisms despite the fact that they are supposed to have a different biochemistry from the kind we know about today,” says Professor Charles Cockell, of the UK Centre for Astrobiology at Edinburgh University. “It is really quite unlikely,” adds Cockell, whose centre will be officially opened this week at a ceremony in Edinburgh.

Ways need to be found to determine whether or not the shadow biosphere exists, says Dimitar Sasselov, professor of astronomy at Harvard University and director of the Harvard Origins of Life Initiative. “If you want a clue you can count up the amount of carbon that is emitted by living things – cows, sheep, grass, plants, forests and all the planet’s bacteria. When you do, you find there is a discrepancy of around 5% when you compare the amount given off from Earth’s standard biosphere and the amount you find in the atmosphere.”

In other words, there is slightly too much carbon dioxide in the atmosphere than can be explained by the emissions of standard lifeforms on Earth. There could be an error in these calculations, of course. Alternatively, the shadow biosphere could be responsible for this excess, says Sasselov. “There is plenty of room for a shadow biosphere. That is clear. Certainly, it is not true, as some allege, that we have strong evidence to show that it does not exist. In fact, the opposite is true: we do not have good enough evidence to dismiss it.”

A key point to note is that scientists – although describing the inhabitants of the shadow biosphere as weird – still assume they will be carbon-based entities. Complex chemistry based on other elements, such as silicon, is possible, they acknowledge but these alternatives cannot create the vast range of organic materials that carbon can generate. In other words, the shadow biosphere, if it exists, will almost certainly be inhabited by carbon life, albeit of an alien variety.

“Billions of years ago, life based on different types of carbon biochemistry could have arisen in several places on Earth,” says Cleland. “These varieties would have been based on different combinations of bases and amino acids. Eventually, one – based on DNA and on proteins made from 20 amino acids – formed multicellular entities and became the dominant form of life on Earth. That is why we find that life as we know it, from insects to humans and from plants to birds, has DNA as its genetic code. However, other lifeforms based on different bases and proteins could still have survived – in the shadow biosphere.”

A different prospect is highlighted by Sasselov, who points out that a complex organic chemical can come in two different shapes even though they have the same chemical formula. Each is a mirror-image of the other and are said to have a different chirality. “Amino acids are an example,” says Sasselov. “Each comes in a right-handed version and a left-handed version. Our bodies – in common with all other lifeforms – only use left-handed versions to create proteins. Right-handed amino acids are simply ignored by our bodies. However, there may be some organisms, somewhere on the planet, that use only right-handed amino acids. They could make up the weird life of the shadow biosphere.”

But how can scientists pinpoint this weird life? Microbes are usually detected in laboratories by feeding nutrients to suspected samples so they grow and expend. Then the resulting cultures can be analysed. A weird lifeform – such as one made only of proteins formed out of right-handed amino acids – will not respond to left-handed nutrients, however. It will fail to form cultures and register its existence.

One solution to this problem is being pursued by Sasselov and colleagues’ Harvard Origins of Life Initiative. They are building an artificial cell – or bionic system – made only of right-handed components including right-handed DNA and right-handed ribosomes. “If there are right-handed lifeforms out there, many of them will be viruses – which will attempt to hijack the DNA of our bionic cells,” adds Sasselov. “When they do that they will leave evidence of their existence. Essentially we are building honey traps to catch any right-handed viruses that might live in the shadow biosphere and so reveal their existence.”

Other scientists suggest a different approach – by looking at Earth’s most inhospitable ecological niches: hot vents on the seafloor, mountaintops, highly saline lakes, Antarctic ice sheets and deserts. Standard lifeforms, mainly bacteria, have been found in these places but only a few. Some niches, researchers speculate, may prove to be just too inhospitable for standard life but may just be tolerable enough to support weird life. Microscopic studies would reveal their existence while standard culture tests would show they had a different biochemistry from standard lifeforms.

Stripes of desert varnish line the canyon walls of Capitol Gorge in Utah. No laboratory has been able to re-create the phenomenon. Photograph: Larry Geddis/Alamy

And a promising example is provided by the desert varnish proposed as a target by Cleland and backed by David Toomey in Weird Life. “No laboratory microbiologist has been able to coax bacteria or algae to make desert varnish,” he states. “It is also possible that the stuff is the end result of some very weird chemistry but no one has been able to reproduce that either.” So yes, these sites could provide proof of the shadow biosphere’s existence, he argues.

Not surprisingly, Cleland agrees. “The only trouble is that no one has yet got round to investigating desert varnish for weird life,” adds Cleland. “I confess I find that disappointing.”

Robin McKie | TheObserver


The views and opinions expressed in this article are those of the authors/source and do not necessarily reflect the position of CSGLOBE or its staff.

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