The New York Times reports:

Life on planet Earth is a riot of diversity: ants and elephants, frogs and blue whales, tulips and redwoods, even a branch of the great apes known as humans. Uniting it all is biochemistry, in particular the shared biochemistry of genetic inheritance — DNA and RNA, the universal instruction kits of life.

Tracing these biochemical threads back through time has convinced many scientists that all life descends from a so-called Last Universal Common Ancestor, or LUCA, that emerged between 4 billion and 4.5 billion years ago, after Earth itself congealed from the gas and detritus of a nascent solar system. How this LUCA rose from the primordial stew may be impossible to ever say definitively. No fossils exist to provide a window onto that moment in time, and scientists have no point of comparison: So far, Earth is the only place in the universe known to harbor life.

After that initial “how,” the questions become marginally easier for scientists to address. Here’s what’s known about three of the biggest inflection points in the evolution of life on Earth.

Scientists agree on little regarding the origins of life, including where it might have arisen — at Earth’s surface, or in its watery depths.

“It’s hard to answer these questions,” said Joanna Masel, a professor of evolutionary biology at the University of Arizona. “The evidence is sketchy. So people argue.”

On one side is the primordial soup hypothesis: Life originated from natural chemical reactions in Earth’s primal waters, which formed a broth of organic compounds.

Charles Darwin floated the idea in 1871, when he proposed that “some warm little pond” was responsible for life’s origin. A few simple chemicals and sources of energy, he suggested, such as natural heat and electricity, might have joined together to form molecules “ready to undergo still more complex changes.” These in turn would have led to the first life form: a tiny sack of chemicals able to divide and make copies of itself.

The hypothesis got its first serious test in 1953. Stanley L. Miller, then a graduate student at the University of Chicago, filled a flask with water as well as ammonia, methane and hydrogen, gases that were considered to be abundant in the early atmosphere.

Then, to mimic a lightning strike, he introduced sparks. Days later, the simple molecules had given rise to amino acids, the building blocks of proteins and virtually all planetary life.

But the amino acids never advanced in complexity, and excitement ebbed as scientists rethought the composition of the primordial air. Some years later, researchers detected amino acids in meteorites, which suggested that the building blocks of life may have rained down from the heavens, putting another potential dent in the primordial soup theory. [Continue reading…]