The mysterious origins of life on Earth – Luka Seamus Wright

Billions of years ago
on the young planet Earth simple organic compounds assembled
into more complex coalitions that could grow and reproduce. They were the very first life on Earth, and they gave rise to every one
of the billions of species that have inhabited our planet since. At the time, Earth was almost completely
devoid of what we’d recognize as a suitable
environment for living things. The young planet had widespread
volcanic activity and an atmosphere that created
hostile conditions. So where on Earth could life begin? To begin the search for
the cradle of life, it’s important to first understand the
basic necessities for any life form. Elements and compounds essential to life
include hydrogen, methane, nitrogen, carbon dioxide, phosphates, and ammonia. In order for these ingredients to comingle
and react with each other, they need a liquid solvent: water. And in order to grow and reproduce, all life needs a source of energy. Life forms are divided into two camps: autotrophs, like plants, that generate
their own energy, and heterotrophs, like animals, that
consume other organisms for energy. The first life form wouldn’t have had
other organisms to consume, of course, so it must have been an autotroph, generating energy either from the sun
or from chemical gradients. So what locations meet these criteria? Places on land or close to the surface
of the ocean have the advantage of access to sunlight. But at the time when life began,
the UV radiation on Earth’s surface was likely too harsh for life
to survive there. One setting offers protection
from this radiation and an alternative energy source: the hydrothermal vents that wind across
the ocean floor, covered by kilometers of seawater
and bathed in complete darkness. A hydrothermal vent is a fissure
in the Earth’s crust where seawater seeps into magma
chambers and is ejected back out
at high temperatures, along with a rich slurry of minerals
and simple chemical compounds. Energy is particularly concentrated at the steep chemical gradients
of hydrothermal vents. There’s another line of evidence
that points to hydrothermal vents: the Last Universal Common Ancestor
of life, or LUCA for short. LUCA wasn’t the first life form,
but it’s as far back as we can trace. Even so, we don’t actually know what
LUCA looked like— there’s no LUCA fossil, no modern-day
LUCA still around— instead, scientists identified genes that
are commonly found in species across all three domains
of life that exist today. Since these genes are shared across
species and domains, they must have been inherited from
a common ancestor. These shared genes tell us that LUCA lived
in a hot, oxygen-free place and harvested energy from a chemical
gradient— like the ones at hydrothermal vents. There are two kinds of hydrothermal vent: black smokers and white smokers. Black smokers release acidic,
carbon-dioxide-rich water, heated to hundreds of degrees Celsius
and packed with sulphur, iron, copper, and other metals essential to life. But scientists now believe that black
smokers were too hot for LUCA— so now the top candidates for the
cradle of life are white smokers. Among the white smokers, a field of hydrothermal vents on the
Mid-Atlantic Ridge called Lost City has become the most favored candidate
for the cradle of life. The seawater expelled here is highly
alkaline and lacks carbon dioxide, but is rich in methane and offers
more hospitable temperatures. Adjacent black smokers may have
contributed the carbon dioxide necessary for life to evolve at Lost City, giving it all the components to support
the first organisms that radiated into the incredible
diversity of life on Earth today.


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