Where does carbon come from originally?
When the carbon-rich cornucopia we call Earth got its act together four-and-a-half billion years ago, it had relatively little of the life-giving element lying around, scientists say. Carbon only arrived in useful quantities over the millennia as a result of a steady bombardment by comets and asteroids. But where did they get their carbon? And what about the sun and Jupiter, both of which are carbon-rich. Where did this life-giving element come from and how did it get here?
To answer these questions, we have to go back to the origin of the universe. In the beginning, according to British scientist Stephen Hawking, there was no carbon. When the Big Bang created the universe approximately fourteen billion years ago, the explosion was so phenomenally hot that all matter existed solely as protons and neutrons. Then, a minute after the Big Bang, when the temperature dropped to ‘only’ 1 billion degrees, some neutrons began to decay into hydrogen while others collided with protons, creating helium. These were the two original elements in the universe. Fast forward two billion years and vast gravitational forces have slowed the expanding universe down, causing large quantities of hydrogen and helium to coalesce into what eventually became galaxies and stars. Some of these stars were extremely hot and would have burned their hydrogen and helium into heavier elements, such as carbon, oxygen and iron – a process that can take only a few hundred million years. When these stars exploded as supernovas, as many eventually did, they ejected their heavy elements into space – to create the raw material for the next generation of stars.
Recent research, in fact, now suggests that our Sun came into existence as the result of a localized ‘Little Bang’ – nine billion years after the Big One. Scientists believe that an exploding supernova created a dense cloud of gas and dust that eventually birthed the Sun and the solar system, thanks again to the work of intense gravitational forces. It’s not likely, however, that the cloud contained much carbon, due to the intense heat generated by the explosion. Carbon arrived later, likely when nearby Red Giant stars blew off their outer, carbon-rich atmospheres, before collapsing into White Dwarfs. This carbon traveled through space mostly as stardust, riding interstellar winds, before coalescing into comets, asteroids, and small planets. This stardust carbon eventually found its way to Earth, and eventually into us.
There are, by the way, carbon stars, which typically are older stars whose atmosphere contain more carbon than oxygen, often giving them a red appearance. In contrast, our Sun is an “ordinary” star, meaning it is richer in oxygen than carbon, which accounts for its yellowish color.
Current thinking among scientists is that most of the carbon on the Earth’s surface arrived in a rain of comets early in its history, not long after it cooled. That’s because the early solar system was a chaotic place, with all sorts of interstellar objects whizzing around. These early bombardments may have delivered virtually all the carbon and other organic material that we can see today on Earth. Some researchers believe that more than half of Earth’s carbon may have been brought in by one massive comet, demonstrating how a single event can change the entire history of a planet.
Here’s a carbon-rich remnant of a supernova explosion called the Crab Nebula:
That’s how carbon made it Earth, but the really interesting question is how life actually began. In other words, how did chemistry (carbon, iron, silicon etc) become biology (DNA, RNA)? According to Stephen Hawking, we don’t really know. Chemists say that the chances of a DNA molecule arising by random events from non-biological material are very small. It’s equally unlikely that a molecule arrived on a comet, Hawking says, because DNA can’t survive for very long when exposed to so much radiation. We do know, thanks to fossil evidence, that life existed here about three and a half billion years ago, or only 500 million years after the Earth became cool enough for life to develop. This suggests that there’s a good chance of the spontaneous generation of life in suitable conditions. Maybe there was some an early form of RNA, which eventually built up to DNA. We can’t say for certain, however, because scientists haven’t yet been able to create life from non-living materials in the laboratory. It’s possible, of course, they just don’t know how.
That’s been the conventional wisdom. However, recently scientists have reexamined the ‘extraterrestrial origin’ idea. In 2009, NASA announced that it had found glycine, one of the fundamental chemical building blocks of life, in a comet for the first time. The amino acid was detected in the dust of Comet Wild-2 in 2004 and returned to Earth by the Stardust probe in 2006. Dr. Carl Pilcher, who leads NASA’s Astrobiology Institute, said “The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the Universe may be common rather than rare.”
In 2008, organic compounds found in the Murchison meteorite suggested that RNA molecules were formed extraterrestrially. Ditto with a study published in 2011 that said DNA molecules, found in meteorites, indicted that they were made in outer space. And within the last year, scientists said that cosmic dust found throughout the universe contains complex organic compounds that could have been created by stars – under much harsher conditions than what existed on Earth during its formative stages. As one of the scientists put it, these organics could have served as the basic ingredients for life. And lastly, in August, 2012, astronomers reported the detection of a sugar molecule in a distant star system for the first time. According to an announcement, this finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.
Such as ours.
Whatever happened, it’s clear that the odds were incredibly long that life would gain a purchase on Earth. Somehow, it did, luckily for us. And carbon was key.
Here’s a fanciful rendition of a asteroid striking the Earth: