fossils of algae from Western Australia. But recent research has shown that the âfossilsâ are far more recent, and in any case may not be fossils at all, but crystals. A 2012 study announced that fossils of bacterial ecosystems dating back 3.49 billion years had been discovered in the Pilbara region of Western Australia, and this is now widely accepted as the oldest evidence of life. 2
Charles Darwin famously speculated that life began in a âshallow, sun-warmed pondâ. But back when Earth formed, its surface was probably covered entirely, or almost so, by oceans. And because Earth lacked an ozone layer for the first 2 billion years of its existence, it is unlikely that shallow waters could have hosted lifeâs origin because ultraviolet radiation would have torn apart the delicate, assembling RNA.
Currently favoured candidates for an earthly origin of life range from hot springs to mid-ocean ridge vents known as âblack smokersâ. Conditions there may have aided the formation of the ever-longer strings of amino acids and molecules, including RNA, that were eventually able to metabolise and reproduce. Mid-oceanic ridges are protected from ultraviolet radiation by the overlying ocean. They are also rich in the elements required for DNA. Additionally, the majority of the most ancient life forms on Earth are thermophiles, small organisms some of which thrive in near-boiling water. One problem for this theory is that water attacks and breaks up the nucleic acid polymers that make up RNA. And unless protected, it is also destabilised by heat.
*
Most research focuses on a search for the earliest life. But perhaps we should be searching instead for evidence of the first nanomachines. Chemical signatures in rocks that result from the activities of the nanomachines offer one means of doing this. For example, studies show that the nanomachines that make atmospheric nitrogen, and can add oxygen to the ammonia so produced in order to create nitrate, were in existence by at least 2.5 billion years ago. 3
Joe Kirschvink argues that Earthâs rocks are the wrong place to look for the nanomachinesâ origins. He is a leading proponent of the seemingly radical theory that the nanomachines, and perhaps life itself, originated on the ice caps and glaciers of ancient Mars. The case is fleshed out fully in A New History of Life , and recent discoveries are building an impressive body of supporting evidence. NASAâs Curiosity lander, for example, has found evidence for ancient Martian streams and ponds: billions of years ago Mars probably had an ocean, as well as land and ice caps. The red planet may have offered a far less hostile environment for assembling naked strings of RNA than Earth. Kirschvink also points out that space travel by early life is not improbable. Mars is small, so its gravity is weak compared with that of Earth. Asteroids could therefore have thrown up a lot of rocks capable of escaping Martian gravity. And we know, through experiments, that meteorites originating from Mars can reach Earth without being sterilised.
But if the nanomachines did originate on Mars, where might they have crossed the âDarwinian thresholdâ and become truly living things? Kirschvink argues that Earthâs atmosphere offers a plausible nursery. Held aloft by fierce winds and currents, the Martian RNA fragments may have mixed with each other, exchanging fragments from one chain to another. Natural selection would have favoured the more functionally complex and efficient strands, which would then have proliferated. Eventually, perhaps when the strands became encompassed by cell walls made of tiny droplets of lipids (a type of molecule that includes fats and waxes), the mass transfer of genes between the nascent nanomachines slowed and their chemistry stabilised.
The Nobel laureate Christian de Duve believed that at this point life would have emerged from nonlife very quickly, perhaps in minutes.
Charles Darwin famously speculated that life began in a âshallow, sun-warmed pondâ. But back when Earth formed, its surface was probably covered entirely, or almost so, by oceans. And because Earth lacked an ozone layer for the first 2 billion years of its existence, it is unlikely that shallow waters could have hosted lifeâs origin because ultraviolet radiation would have torn apart the delicate, assembling RNA.
Currently favoured candidates for an earthly origin of life range from hot springs to mid-ocean ridge vents known as âblack smokersâ. Conditions there may have aided the formation of the ever-longer strings of amino acids and molecules, including RNA, that were eventually able to metabolise and reproduce. Mid-oceanic ridges are protected from ultraviolet radiation by the overlying ocean. They are also rich in the elements required for DNA. Additionally, the majority of the most ancient life forms on Earth are thermophiles, small organisms some of which thrive in near-boiling water. One problem for this theory is that water attacks and breaks up the nucleic acid polymers that make up RNA. And unless protected, it is also destabilised by heat.
*
Most research focuses on a search for the earliest life. But perhaps we should be searching instead for evidence of the first nanomachines. Chemical signatures in rocks that result from the activities of the nanomachines offer one means of doing this. For example, studies show that the nanomachines that make atmospheric nitrogen, and can add oxygen to the ammonia so produced in order to create nitrate, were in existence by at least 2.5 billion years ago. 3
Joe Kirschvink argues that Earthâs rocks are the wrong place to look for the nanomachinesâ origins. He is a leading proponent of the seemingly radical theory that the nanomachines, and perhaps life itself, originated on the ice caps and glaciers of ancient Mars. The case is fleshed out fully in A New History of Life , and recent discoveries are building an impressive body of supporting evidence. NASAâs Curiosity lander, for example, has found evidence for ancient Martian streams and ponds: billions of years ago Mars probably had an ocean, as well as land and ice caps. The red planet may have offered a far less hostile environment for assembling naked strings of RNA than Earth. Kirschvink also points out that space travel by early life is not improbable. Mars is small, so its gravity is weak compared with that of Earth. Asteroids could therefore have thrown up a lot of rocks capable of escaping Martian gravity. And we know, through experiments, that meteorites originating from Mars can reach Earth without being sterilised.
But if the nanomachines did originate on Mars, where might they have crossed the âDarwinian thresholdâ and become truly living things? Kirschvink argues that Earthâs atmosphere offers a plausible nursery. Held aloft by fierce winds and currents, the Martian RNA fragments may have mixed with each other, exchanging fragments from one chain to another. Natural selection would have favoured the more functionally complex and efficient strands, which would then have proliferated. Eventually, perhaps when the strands became encompassed by cell walls made of tiny droplets of lipids (a type of molecule that includes fats and waxes), the mass transfer of genes between the nascent nanomachines slowed and their chemistry stabilised.
The Nobel laureate Christian de Duve believed that at this point life would have emerged from nonlife very quickly, perhaps in minutes.
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