48 ■ CHAPTER 03 Chemistry of Life
CELLS
Although proteins use only 20 amino acids,
scientists have identified 90 amino acids in
meteorites, suggesting that the first organic
materials could have come to Earth from outer
space. At NASA’s Ames Research Center in
California, astrophysicist Scott Sandford and
colleagues have been able to make amino acids
from gases using conditions like those that exist
in interstellar space.
In several experiments, Sandford’s team froze
different combinations of gases to simulate
ices found on comets and in interstellar clouds
(where new stars and planets form), and then
bombarded those ices with radiation. And what
do their icy-hot recipes create? “We make amino
acids all the time,” says Sandford. In many cases,
they identified amino acids similar to those
that Miller’s experiment created. “The universe
seems to be an organic chemist,” says Sandford.
“It is hardwired to turn simple molecules into
more complicated ones. The consequence of that
is that it makes a whole host of products, some of
which are biologically interesting.”
On October 7, 2008, an asteroid entered
Earth’s atmosphere over Africa, and a NASA
satellite photographed its impact (Figure 3.8).
Initially 6–15 feet in diameter, the asteroid
exploded over eastern Africa, and its fragments
landed in Sudan. NASA scientists were able to
find pieces of the asteroid on the ground because
the dark rocks stood out against the light-
colored desert sand. These fragments are called
the “Almahata Sitta” or “Station Six” meteorites,
named after a train station near the location
where pieces were recovered.
When the fragments were analyzed, scientists
found 19 different amino acids. The extraterres-
trial origin of these amino acids is confirmed by
a particular characteristic of amino acid mole-
cules: They have two forms, left-handed and
right-handed, that are mirror images of each
other. All of the amino acids produced by living
organisms on Earth are of the left-handed form;
right-handed amino acids are produced only in
laboratories. The amino acids in the Almahata
Sitta meteorites are a mixture of left-handed
and right-handed forms. The presence of right-
handed amino acids shows that the amino acids
in the fragments really came from space, not
from terrestrial organic contamination after the
fragments landed.sulfide today—coinciding with lightning, which
is often focused around volcanoes, may have
played a role in making large and varied quanti-
ties of biologically crucial molecules, setting the
stage for the evolution of life on Earth. But this
is not the only hypothesis about the origins of
life. There are other ideas, including one that has
gained traction in recent years: that the building
blocks for life arrived from space.Figure 3.8
Asteroid 2008 TC3 contains amino acids that formed in
outer space
Q1: How did the NASA scientists find the fragments of the meteorite
that exploded over eastern Africa?Q2: What piece of evidence suggests that amino acids found in the
meteorite fragments originated in outer space?Q3: Speculate on the significance of finding extraterrestrial amino
acids.A NASA satellite
photographed the
impact of the
asteroid on
October 7, 2008.
The yellow arrow
traces the path the
asteroid followed,
and the reddish-
orange blob
shows the point at
which it exploded
after entering
Earth’s
atmosphere.The asteroid broke into
fist-sized pieces that fell
into the Nubian Desert
in Sudan. The dark color
of the fragments made
them conspicuous on
the desert sand, and
many of the fragments
were recovered by
NASA scientists in
February 2009.