- Michael Marshall*
- BBC Future
Charles Darwin had some pretty good ideas. The most famous is the theory of evolution by natural selection, which explains much of what we know about life on Earth.
But he also reflected on many other questions. In a hasty letter to a friend, he presented an idea about how the first life could have been formed.
Some 150 years later, that letter seems notably ahead of its time, perhaps even prophetic.
Contrary to popular belief, Darwin was not the first to propose that species evolve. The idea that animal populations change over time, for example, that today’s giraffes have longer necks than their distant ancestors, was much debated in the 19th century.
But Darwin’s key contribution was outlining a mechanism of evolution: natural selection.
The idea is that animals of a species compete with each other for food, shelter, and the ability to reproduce. Alone the fittest, that is, those that adapt better to their environment, will be able to reproduce, so his traits will be passed on to the next generation and they will become more common.
So if having a long neck is useful for giraffes, over generations giraffes with longer necks will proliferate until they reach the optimal neck length. Darwin put this out in his 1859 work “On the Origin of Species.”
The fact of evolution implies something about how life began.
Evolution tells us that seemingly distinct species are distant relatives, both descendants of a single shared ancestor. For example, our closest living relatives are chimpanzees: the common ancestor we share lived at least seven million years ago.
Furthermore, each living organism ultimately descends from a single ancestral population: the Last Universal Common Ancestor (LUCA, for its acronym in English), which lived more than 3.5 billion years ago when the planet was formed.
However, the theory of evolution tells us nothing about how the first life formed: it only tells us how and why existing life changes.
How did life begin?
Research on the origin of life didn’t really begin until the 1950s.
By then, many scientists they suspected that life began in the oceans. The idea was that many carbon-based chemicals formed on Earth dissolved in the ocean, which became thick: the so-called “primeval soup.”
This was proposed in the 1920s by a Soviet biologist named Alexander Oparin. In 1953, a young American student named Stanley Miller showed that amino acids, the building blocks of proteins, could be formed in a simple apparatus that mimicked the primordial ocean and atmosphere.
The idea that life began in the ocean prevailed for decades, but there was an obvious problem: the oceans are huge, so unless they occur carbon-based chemicals in staggering amountsThey would be adrift for years and never meet.
“It would have too much water and too few molecules,” says Claudia Bonfio of the MRC Molecular Biology Laboratory in Cambridge, UK.
A much discussed alternative is that life could have started in alkaline vents like those of Lost City in the mid-Atlantic.
There, the hot, mineral-rich water from the sea floor seeps through the rocks and forms mysterious white needles. Vents are a rich source of chemical energy that could have fed the first organisms.
But according to a new study published last May, “the direct synthesis of amino acids or nucleobases”, which are crucial for life as we know it, “has not yet been demonstrated” under alkaline ventilation conditions.
That brings us back to Darwin.
A letter to a friend
Darwin never wrote in his books about how life began, but he speculated about it in private.
The key document is a letter dated February 1, 1871 who wrote to a close friend, the naturalist Joseph Dalton Hooker. This letter is now almost 150 years old.
It is short, only four paragraphs, and difficult to read due to Darwin’s handwriting. After a brief discussion of some recent experiments on mold, Darwin outlined the beginnings of a hypothesis:
“It is often said that all the conditions for the first production of a living being are now present, which might have once been present. But if (and oh, what a great if) we could conceive in some warm little pond with all kinds of ammonia and phosphoric salts, light, heat, electricity present, that a protein compound was chemically formed, ready to undergo even more complex changes, in the present, such matter would be instantly eaten or absorbed, which would not have been the case before that living creatures were formed. “
This requires a bit of unraveling, in part because several ideas are stuck: it reads as if Darwin was thinking of his hypothesis even when he wrote it. But the central idea is quite simple.
Darwin was proposing that life could begin not in the open ocean, but in a smaller body of water on land, which was rich in chemicals.
This is, in essence, the whole idea of the soup, but with an advantage: in a swimming pool, any dissolved chemical substance becomes concentrated when the water evaporates in the heat of the day.
The initial synthesis of the chemicals of life would be driven by some combination of light, heat and chemical energy.
In many ways, Darwin’s idea is hopelessly incomplete, but he cannot be blamed for that. I was writing before the discovery of nucleic acids like DNA, before biologists understood how genes work, and when the inner workings of living cells were very mysterious.
Darwin imagined that life began with a protein, but no one really knew what proteins wereIt was not until 1902 that it was understood that proteins are chains of amino acids.
But the same basic scheme is still followed today, and many researchers are convinced that this is the best explanation we have for the origin of life.
Heat and light
Lena Vincent from the University of Wisconsin-Madison is a researcher whose work is compatible with a pond environment, although she prefers to keep an open mind.
You are trying to create sets of chemicals that copy themselves as a group.
The simplest example would be a pair of chemicals A and B, where each has the ability to produce the other, so A produces B and B produces A. A pair of such chemicals could reproduce by themselves, although neither of them could do it alone.
In practice, chemical assemblies are more complicated than that, but the principle is the same.
There is also much evidence that ultraviolet radiation from sunlight can drive the formation of key biological chemicals, especially RNA, a DNA-like nucleic acid that is believed to be a crucial component in the creation of the first life.
Such processes could only occur in well-lit places, again pointing to a small body of water rather than the depths of the sea.
A protagonist in this has been John Sutherland of the MRC Molecular Biology Laboratory in Cambridge, UK.
In 2009 he showed that two of the four building blocks of RNA are formed from simple carbon-based chemicals, if they undergo simple treatments such as being bathed in ultraviolet radiation.
Since then, he’s shown that the same starter chemicals, with subtly different treatments, too can become the building blocks of proteins or fatty lipids that form the outer membranes of cells.
Finally, bodies of water on land can almost completely dry out in hot weather and then fill up again when it rains. These wet-dry cycles may seem harmless, but they have profound effects on the chemicals of life.
The cradle of civilization
Deamer argues that “fluctuating volcanic hot spring pools” are the most likely setting for the beginning of life.
Sutherland has an alternative: a meteor crater, with streams running down the sides and meeting in a pool at the bottom.
It is not clear which of these scenarios is more plausible. Furthermore, many younger researchers make sure not to commit to one scenario or another, arguing that we still don’t know enough about the processes that can lead to life.
In particular, many researchers still take the alkaline vent hypothesis seriously, despite its problems.
What is clear, however, is that Darwin’s idea was forward-thinking. He envisioned the need to concentrate a variety of chemicals in a small space and the need for an energy source that could drive chemical reactions.
“Like many of Darwin’s ideas,” Vincent says, the little warm pond hypothesis was “very prescient.”
Darwin pointed out another fact in his letter, which is “underestimated,” says Vincent.
“The processes that occur in that little warm pond can occur so easily that would be happening all the time“, he maintains.
We may not see it simply because every time a new protein or similar form forms naturally, it is eaten by hungry bacteria.
“We talk about the origin of life as if it were something that happened in the deep past,” says Vincent. “But is something that could be happening even at this time“.
* Michael Marshall is a science writer living in Devon, UK. He wrote a book on the origins of life, The Genesis Quest.
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