stated clearly presents how to spot an
alien according to NASA's definition of
life NASA and other space agencies
currently have hundreds of satellites
multiple probes high-powered telescopes
Rovers and even a manned space station
out exploring the cosmos many of these
tools are equipped with cameras and some
with chemistry kits able to help us spot
alien life so far the existence of alien
life has not yet been confirmed but
aside from little green men with laser
guns as depicted in sci-fi what might
aliens actually be like if we were to
find a smear of strange goo on Mars for
example would we even be able to tell if
it were alive biologists have been
asking these types of questions not
about aliens but about unusual specimens
found here on earth for well over a
century suppose for a moment that while
exploring in a cave you happen to
discover two cotton white puff balls
apparently growing on the side of a rock
could they be alive fungus perhaps or
maybe real-life tribbles to help solve
these kinds of mysteries scientists have
come up with various list definitions of
life similar to this one here it's a
series of traits that living things
typically all have but that non living
things normally lack these lists help us
ask useful questions about puzzling
specimens question number one do your
mystery creatures grow well maybe two of
them were found and one is larger than
the other
that said storms lakes crystals and
fires can also grow but they don't
really qualify as life now do they time
to ask more questions
can the puffballs reproduce so far you
haven't witnessed it happen but there
are two of them so again maybe that said
fires can also reproduce by sending out
a spark but we don't consider fires to
be alive time to check a third item on
our list are the mystery specimens made
of cells almost all living things are
except for viruses this by the way is
one of several reasons that viruses are
not considered alive by some
biologists a thorough inspection under
the microscope suggests that our
specimen has no cells they might be
buried inside somewhere but the
puffballs themselves though they are
somewhat soft appear to be made of
flexible crystals rather than cells
further testing shows that our specimens
also seem to lack other traits from our
list definition largely convinced that
they are not alive you use a cup of
coffee to bribe a geologist for a free
consultation she informs you that what
you have found are ouka night crystals
your specimens are not living things
while the list definition of life is not
perfect
we've already seen that it has trouble
helping us categorize viruses for
example it is fairly handy for people
that study modern living creatures here
on earth the list definition is not very
useful however for at least two special
groups of researchers EXO biologists
people investigating the possibility of
alien life in the cosmos and prebiotic
chemists researchers looking into the
idea that life on Earth may have
naturally emerged from chemical
reactions prebiotic meaning before life
if aliens do turn out to actually look
like monsters or little green men with
laser guns they shouldn't be too hard to
spot but most life-forms on earth are
microscopic and tricky to detect what if
alien life is - furthermore life-forms
on earth are extremely complex this is
even true for bacteria and archaea the
simplest critters that qualify as life
according to our list definition alien
life and the first true life-forms that
developed on earth may be far simpler
but how much simpler without a clear and
widely accepted definition of life
researchers could waste a lot of time
and effort debating the significance of
a new discovery for example back in the
50s prebiotic chemist Sidney Fox once
produced what he called protein I'd
microspheres tiny orbs that act
superficially like cell membranes
when suspended in a solution full of
other protein odes microspheres can grow
and even reproduce by splitting in two
because they can reproduce and because
they could have easily formed on the
early Earth
after fox initially believed that
microspheres were alive and represent
what the first life-forms may have been
like on our planet
after years of investigation and intense
debate for reasons we'll see here
shortly
researchers today no longer consider
microspheres to be alive
a similar confusion arose when the
Viking probes sent to Mars in 1976
discovered strange chemical reactions
initially thought to be signs of
microbial life while some researchers
still argue that life may have actually
been found by those probes further
research told us we likely just
witnessed unexpected reactions caused by
the strange chemistry on Mars to help
unify our thinking and avoid both false
positives and false negatives in our
search for life in 1994 NASA put
together a council to discuss the
essential properties of living things
the result of that meeting was the
production of a single sentence a
sentence now fixed in the minds of
thousands of researchers and soon to be
fixed in yours
NASA's definition of life life they
proposed is any self-sustaining chemical
system capable of Darwinian evolution
let me say that one more time
life is a self-sustaining chemical
system capable of Darwinian evolution
not all scientists use this definition
some feel it's a little bit too
constraining but if we were to discover
something native to Mars that undeniably
fit this definition it would excite
celebration from even the harshest of
critics they defined life as a chemical
system because all living things we know
of and most all that we can imagine our
made of diverse collections of molecules
and are powered by a flowing tapestry of
chemical reactions saying that life is
self-sustaining means that within a
specific environment living things can
carry on without the supervision of say
a chemist creatures may require distinct
molecules and energy sources from their
specific environments but as long as
those are supplied life maintains itself
under this definition it can be easily
argued that viruses and even individual
genes qualify as life forms from this
perspective the cells that genes and by
live inside can be thought of as their
environments the cell itself of course
is also a self-sustaining chemical
system here we see layers of life inside
of life a fascinating new way to think
about biology finally to be considered
alive a system must be capable of
adapting through Darwinian evolution
this requirement though argued against
by some is extremely important because
Darwinian evolution reproduction with
heritable modifications acted upon by
selection is the most powerful natural
creative process ever to be discovered
it allows organisms to accumulate
adaptations over time and is a process
that all life forms on Earth individual
genes and viruses included have access
to Darwinian evolution is what gave the
eyeball its lens the bird its wings and
the protozoan its flagella for Darwinian
evolution to work a chemical system must
be capable of reproduction within its
environment offspring must be capable of
some form of mutation or modification
and when a modified individual
reproduces its mutations must be
passable to at least some of its
offspring
dr. Fox's microspheres may grow and
reproduce but they cannot pass mutations
to their offspring
this means they do not have access to
Darwinian evolution and because of this
they will not accumulate positive
mutations over time they are not
expected to increase in complexity they
do not qualify as living things it's
important to note that NASA's definition
of life is not written in stone future
discoveries might force us to modify it
or completely rethink it for now however
many researchers find it incredibly
useful as we continue searching for life
in the laboratory and throughout the
cosmos the definition helps unify our
thinking quickly and affordably resolve
debates and it guides the logic in the
experiments we design I'm Jon Perry and
that is life a self-sustaining chemical
system capable of Darwinian evolution
stated clearly
this animation was funded by the Center
for chemical evolution the National
Science Foundation NASA and our very own
viewers our viewers contribute to us on
patreon.com forward slash stated clearly
here you can see a list of some of their
names good people right there good folks
thank you special thanks to all of the
scientists and philosophers that helped
me write this script it really helped me
dig into the science and make sure that
I was teaching things correctly
thank you I could not have done this
without you if you enjoyed this
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so long for now stay curious
you