This suggests humans didn’t originate on Earth — we came from space.
More of the ingredients for life have been found in
meteorites.
Space rocks that fell to Earth within the last century
contain the five bases that store information in DNA and RNA, scientists report in Nature Communications.
These “nucleobases” — adenine, guanine, cytosine, thymine
and uracil — combine with sugars and phosphates to make up the genetic code of
all life on Earth. Whether these basic ingredients for life first came from
space or instead formed in a warm soup of earthly chemistry is still not known. But the discovery adds to evidence that suggests life’s
precursors originally came from space, the researchers say.
Scientists have detected bits of adenine, guanine and other
organic compounds in meteorites since the 1960s.
Researchers have also seen hints of uracil, but cytosine and thymine remained
elusive, until now.
“We’ve completed the set of all the bases found in DNA and
RNA and life on Earth, and they’re present in meteorites,” says astrochemist
Daniel Glavin of NASA’s Goddard Space Flight Center in Greenbelt, Md.
A few years ago, geochemist Yasuhiro Oba of Hokkaido
University in Sapporo, Japan, and colleagues came up with a technique to gently
extract and separate different chemical compounds in liquified meteorite dust
and then analyze them.
“Our detection method has orders of magnitude higher
sensitivity than that applied in previous studies,” Oba says. Three years ago,
the researchers used this same technique to discover ribose, a sugar needed for
life, in three meteorites.
In the new study, Oba and colleagues combined forces with
astrochemists at NASA to analyze one of those three meteorite samples and three
additional ones, looking for another type of crucial ingredient for life:
nucleobases.
The researchers think their milder extraction technique,
which uses cold water instead of the usual acid, keeps the compounds intact.
“We’re finding this extraction approach is very amenable for these fragile
nucleobases,” Glavin says. “It’s more like a cold brew, rather than making hot
tea.”
With this technique, Glavin, Oba and their colleagues
measured the abundances of the bases and other compounds related to life in
four samples from meteorites that fell decades ago in Australia, Kentucky and
British Columbia. In all four, the team detected and measured adenine, guanine,
cytosine, uracil, thymine, several compounds related to those bases and a few
amino acids.
Using the same technique, the team also measured chemical
abundances within soil collected from the Australia site and then compared the
measured meteorite values with that of the soil. For some detected compounds,
the meteorite values were greater than the surrounding soil, which suggests
that the compounds came to Earth in these rocks.
But for other detected compounds, including cytosine and
uracil, the soil abundances are as much as 20 times as high as in the
meteorites. That could point to earthly contamination, says cosmochemist
Michael Callahan of Boise State University in Idaho.
“I think [the researchers] positively identified these
compounds,” Callahan says. But “they didn’t present enough compelling data to
convince me that they’re truly extraterrestrial.” Callahan previously worked at
NASA and collaborated with Glavin and others to measure organic materials in
meteorites.
But Glavin and his colleagues point to a few specific
detected chemicals to support the hypothesis of an interplanetary origin. In
the new analysis, the researchers measured more than a dozen other life-related
compounds, including isomers of the nucleobases, Glavin says. Isomers have the
same chemical formulas as their associated bases, but their ingredients are
organized differently. The team found some of those isomers in the meteorites
but not in the soil. “If there had been contamination from the soil, we should
have seen those isomers in the soil as well. And we didn’t,” he says.
Going directly to the source of such meteorites — pristine asteroids — could clear up the matter. Oba and colleagues are already using their extraction technique on pieces from the surface of the asteroid Ryugu, which Japan’s Hayabusa2 mission brought to Earth in late 2020. NASA’s OSIRIS-REx mission is expected to return in September 2023 with similar samples from the asteroid Bennu.
“We’re really excited about what stories those materials have to tell,” Glavin says.