An artist's illustration of early life on Earth.
CREDIT: Peter Sawyer / Smithsonian Institution
All life on Earth relies on a standard set of 20 molecules called amino acids to build the proteins that carry out life's essential actions. But did it have to be this way?
All living creatures on this planet use the same 20 amino acids, even though there are hundreds available in nature. Scientists therefore have wondered if life could have arisen based on a different set of amino acids.
And what's more, could life exist elsewhere that utilizes an alternate collection of building blocks? [5 Bold Claims of Alien Life]
"Life has been using a standard set of 20 amino acids to build proteins for more than 3 billion years," said Stephen J. Freeland of the NASA Astrobiology Institute at the University of Hawaii. "It's becoming increasingly clear that many other amino acids were plausible candidates, and although there's been speculation and even assumptions about what life was doing, there's been very little in the way of testable hypotheses."
So Freeland and his University of Hawaii colleague Gayle K. Philip devised a test to try to learn if the 20 amino acids Earth's life uses were randomly chosen, or if they were the only possible ones that could have done the job.
"We found that chance alone would be extremely unlikely to pick a set of amino acids that outperforms life's choice," Freeland said.
In fact, the researchers think early life on Earth probably used a version of natural selection to choose these amino acids. Some combinations of other amino acids were likely tried, but none proved quite as fit, so no other combinations ended up producing the numbers of successful offspring that the existing set achieved.
"Here we found a very simple test that begins to show us that life knew exactly what it was doing," Freeland said. "This is consistent with the idea that there was natural selection going on."
Getting at the question of why nature chose the 20 amino acids it did is experimentally difficult, said Aaron Burton, a NASA Postdoctoral Program Fellow who works as an astrochemist at NASA's Goddard Space Flight Center in Greenbelt, Md.
Right now the race is on to directly find amino acids elsewhere in the solar system. Some hints that they abound have been found on meteorites that have landed on Earth from outer space, as well as from missions such as NASA's Stardust probe, which sampled the coma of comet Wild 2 in 2004.
"All signs are that amino acids are going to be found throughout the galaxy," Freeland said. "They are apparently obvious building blocks with which to construct life. What we're finding hints at a certain level of predictability in the way things turned out."
The question of life's amino acid toolbox is interesting not just in trying to trace the origin of the life on Earth, but in wondering whether life exists on other planets, and if so, what form it takes. Scientists are particularly curious about how a different set of amino acid building blocks would result in different characteristics in the life it creates.
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From The Urantia Book:
58:1.3 The Satania Life Carriers had projected a sodium chloride pattern of life; therefore no steps could be taken toward planting it until the ocean waters had become sufficiently briny. The Urantia type of protoplasm can function only in a suitable salt solution. All ancestral life—vegetable and animal—evolved in a salt-solution habitat. And even the more highly organized land animals could not continue to live did not this same essential salt solution circulate throughout their bodies in the blood stream which freely bathes, literally submerses, every tiny living cell in this “briny deep.”
58:4.1 That we are called Life Carriers should not confuse you. We can and do carry life to the planets, but we brought no life to Urantia. Urantia life is unique, original with the planet. This sphere is a life-modification world; all life appearing hereon was formulated by us right here on the planet; and there is no other world in all Satania, even in all Nebadon, that has a life existence just like that of Urantia.
550,000,000 years ago the Life Carrier corps returned to Urantia. In co-operation with spiritual powers and superphysical forces we organized and initiated the original life patterns of this world and planted them in the hospitable waters of the realm. All planetary life (aside from extraplanetary personalities) down to the days of Caligastia, the Planetary Prince, had its origin in our three original, identical, and simultaneous marine-life implantations. These three life implantations have been designated as: the central or Eurasian-African, the eastern or Australasian, and the western, embracing Greenland and the Americas.
500,000,000 years ago primitive marine vegetable life was well established on Urantia. Greenland and the arctic land mass, together with North and South America, were beginning their long and slow westward drift. Africa moved slightly south, creating an east and west trough, the Mediterranean basin, between itself and the mother body. Antarctica, Australia, and the land indicated by the islands of the Pacific broke away on the south and east and have drifted far away since that day.
We had planted the primitive form of marine life in the sheltered tropic bays of the central seas of the east-west cleavage of the breaking-up continental land mass. Our purpose in making three marine-life implantations was to insure that each great land mass would carry this life with it, in its warm-water seas, as the land subsequently separated. We foresaw that in the later era of the emergence of land life large oceans of water would separate these drifting continental land masses.