Nous publions ci-dessous un article en anglais complétant utilement celui de la veille
WHEN Earth formed 4.5 billion years ago, it was a sterile ball of rock, slammed by meteorites and carpeted with erupting volcanoes. Within a billion years, it had become inhabited by microorganisms. Today, life covers every centimetre of the planet, from the highest mountains to the deepest sea. Yet, every other planet in the solar system seems lifeless. What happened on our young planet? How did its barren rocks, sands and chemicals give rise to life?
Many ideas have been proposed to explain how life began. Most are based on the assumption that cells are too complex to have formed all at once, so life must have started with just one component that survived and somehow created the others around it. When put into practice in the lab, however, these ideas don’t produce anything particularly lifelike. It is, some researchers are starting to realise, like trying to build a car by making a chassis and hoping wheels and an engine will spontaneously appear.
The alternative – that life emerged fully formed – seems even more unlikely. Yet perhaps astoundingly, two lines of evidence are converging to suggest that this is exactly what happened. It turns out that all the key molecules of life can form from the same simple carbon-based chemistry. What’s more, they easily combine to make startlingly lifelike “protocells”. As well as explaining how life began, this “everything-first” idea of life’s origins also has implications for where it got started – and the most likely locations for extraterrestrial life, too.
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The problem with understanding the origin of life is that we don’t know what the first life was like. The oldest accepted fossils are 3.5 billion years old, but they don’t help much. They are found in ancient rock formations in Western Australia known as stromatolites and are single-celled microorganisms like modern bacteria. These are relatively complex: even the simplest modern bacteria have more than 100 genes. The first organisms must have been simpler. Viruses have fewer genes, but can reproduce only by infecting cells and taking them over, so can’t have come first.
What is the definition of life?
With physical evidence lacking, origin-of-life researchers begin by asking two questions. What are the fundamental processes underpinning life? And what chemicals do these processes use? Here, there are answers.
Life can be boiled down to three core systems. First, it has structural integrity: that means each cell has an outer membrane holding it together. Second, life has metabolism, a set of chemical reactions that obtain energy from its surroundings. Finally, life can reproduce using genes, which contain instructions for building cells and are passed on to offspring.
Biochemists know the chemicals underpinning these processes too. Cell membranes are made of lipids, molecules containing long chains of carbon atoms. Metabolism is run by proteins – chains of amino acids, twisted into pretzel shapes – especially enzymes, which help catalyse chemical reactions, speeding them up. And genes are encoded in molecules called nucleic acids, such as deoxyribonucleic acid, better known as DNA.
A billion years after Earth formed, life emerged. Did it happen elsewhere too?
Beyond this, things start to become more complicated. Life’s three core processes are intertwined. Genes carry instructions for making proteins, which means proteins only exist because of genes. But proteins are also essential for maintaining and copying genes, so genes only exist because of proteins. And proteins – made by genes – are crucial for constructing the lipids for membranes. Any hypothesis explaining life’s origin must take account of this. Yet, if we suppose that genes, metabolism and membranes were unlikely to have arisen simultaneously, that means one of them must have come first and “invented” the others.
An early idea put proteins in the driving seat. In the 1950s, biochemist Sidney Fox discovered that heating amino acids made them link up into chains. In other words, they formed proteins, albeit with a random sequence of amino acids rather than one determined by a genetic code. Fox called them “proteinoids” and found that they could form spheres, which resembled cells, and catalyse chemical reactions. However, the proteinoids never got much further. Some researchers still hunt for lifelike behaviour in simple proteins, but the idea that proteins started life on their own has now been largely rejected.
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More recently, much research has focused on an idea called the RNA world. Like DNA, RNA (ribonucleic acid) carries genes. The discovery that some kinds of RNA can also catalyse chemical reactions hinted that the first RNA molecules could have been enzymes that made copies of themselves and so got life started. However, biochemists have spent decades struggling to get RNA to self-assemble or copy itself in the lab, and now concede that it needs a lot of help to do either.
Perhaps, then, membranes came first. David Deamer at the University of California, Santa Cruz, has championed this option. In the 1970s, his team discovered that lipids found in cell membranes could be made when two simple chemicals, cyanamide and glycerol, were mixed with water and heated to 65°C. If these lipids were subsequently added to salt water and shaken, they formed spherical blobs with two outer layers of lipids, just like cells. “The simplest function is the self-assembly of membranes. It’s spontaneous,” says Deamer. Nevertheless, he now accepts that this isn’t enough, because lipids can’t carry genes or form enzymes.
The shortcomings of these simple models of life’s origin have led Deamer and others to explore the seemingly less plausible alternative that all three systems emerged together in a highly simplified form.
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Remarque
Les dernières observations de l’espace profond faites par le télescope spatial américain James Webb font supposer que l’univers visible contient
plus de mille milliards de galaxies. Les planètes habitables de ces galaxies hébergent elles des formes de vie identiques ou différentes ?
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