The origins of two of the five molecular subcomponents of the human genetic code are still a mystery to scientists, according to a new report.
Researchers Ben K.D. Pearce and Ralph Pudritz wrote in a paper published in Astrobiology they are perplexed as to where two key parts of RNA and DNA came from.
The pair reported that the other three components — guanine, adenine, and uracil — likely came from a meteorite.
"Cytosine (C) and thymine (T) however are absent in meteorites, making the emergence of a RNA and later RNA/DNA/protein world problematic," they wrote.
Forbes reported on the findings.
"But we simply don't know where cytosine came from," the researchers wrote. "There is no compelling evidence as of yet for forming cytosine in situ on the nascent Earth."
In recent years, scientists have attempted to unravel humans' genetic blueprint via the Human Genome Project.
Another project, called the Human Cell Atlas, is trying to map out every cell in the body.
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