4.6 billion years ago our Earth coalesced from protoplanetary materials
revolving around the Sun. The period following the formation of the Earth was
extreme. Radioactivity and constant bombardment by asteroids made our planet
hot. When the atmosphere first formed, it was mostly comprised of hydrogen, and
shortly after, probably contained nitrogen, ammonia, methane, water vapor, and
carbon dioxide. Unlike in today's atmosphere there was
virtually no oxygen available on the early Earth. It was during this extreme
period when life first took hold on our planet. Today, we have incredible
biological diversity in different habitats around the globe, but in places
like Yellowstone National Park we can find particular environments that mimic
the conditions of the early Earth. Yellowstone is like a window into the
past, the very distant past. I'm Luke McKay and I'm an astrobiologist who
studies extreme environments to understand how life began on Earth and
where else beyond earth life could exist. I grew up playing in the forests and
rivers of Alabama and I was obsessed with TV shows like the Crocodile Hunter.
This translated to being fascinated by biology and exotic environments,
especially when the boundaries of life are pushed well beyond what is possible
for humans. There are over 14,000 thermal features in Yellowstone National Park.
Some of them are nearly identical to one another, while others are extraordinarily
unique based on their specific chemical conditions and geological environments.
Depending primarily on the pH, temperature, and oxygen or sulfide
concentrations, the fumaroles, mudpots, geysers, and hot springs of Yellowstone
yield a massive variety of ecological niches that harbor diverse types of
life. This means that the combination of
microorganisms living in Grand Prismatic spring is different from that in the
Norris Geyser Basin because these two thermal areas are geologically and
chemically distinct. And if we zoom in on only one of these regions there are
multiple local thermal features with diverse types of life. While it is
difficult to know the exact conditions that supported the origin of life on
Earth, we can examine the biology living in
these thermal habitats to find organisms that have deep evolutionary roots and
thrive under extreme conditions. Consequently, extremophiles in
Yellowstone can help us understand how similar organisms may have lived nearly
4 billion years ago. My research has brought me to what I now consider my
favorite feature in Yellowstone: Washburn Hot Springs in the Canyon area.
Washburn Hot Springs is a dark bubbling cauldron with temperatures around 150
degrees Fahrenheit or 65 degrees Celsius and a near neutral pH of 6 and a half.
The gases emanating from this pool are comprised almost entirely of carbon
dioxide, methane, hydrogen, and hydrogen sulfide. While it has little to no oxygen,
Washburn is rich in both carbon and energy, and supports a diverse community
of poorly understood microorganisms. Both the chemical and the biological
characteristics of Washburn Hot Springs suggest it may be an ideal modern-day
analogue of ancient environments where life originated. Recently, my research
colleagues and I examined the genomes of microorganisms thriving in Washburn Hot
Springs for clues about ancient life. Although 4 billion years of evolution
has undoubtedly changed these thermophiles quite a bit, we can compare
DNA from organisms living today to decipher evolutionary histories and
determine last common ancestors. Just as we suspected, we found several
thermophiles in Washburn Hot Springs that likely produce or consume methane
gas, which was exciting because scientists have shown that a
methane-based metabolism was one of the earliest modes of life.
This means that organisms that make methane today, called methanogens, have
deep evolutionary roots that span almost the entire history of life on Earth.
For comparison, apes only date back to 0.3 percent of life on Earth and the
modern human species has only been around for 0.009 percent of living
history. One particular methane-metabolizing
organism in Washburn Hot Springs that caught our attention belonged to a very
interesting taxonomic group called the Korarcheota. DNA comparisons indicated
this is one of the most ancient types of archaea, one of the three domains of life.
In addition to its ability to produce or consume methane, our analyses also
suggested that this Korarcheot could perform a sulfur-based metabolism, which
is thought to be another very ancient way of life. And no organism has
previously been observed to have both methane- and sulfur-based metabolisms.
Because of these interesting characteristics, we provisionally named
this organism Methanodesulfokores washburnensis. Methano for methane,
desulfo for sulfur, kores for Korarcheata, and Washburnensis because, you
guessed it, it came from Washburn Hot Springs in Yellowstone National Park. In
summary, while we cannot expect modern life to be the same as organisms that
lived four billion years ago, we can use DNA from organisms alive today,
specifically those in extreme environments of Yellowstone, to deduce
what early life was like. In turn, uncovering these clues about early life
on Earth helps us understand if life also could
have originated on another planetary body, like Venus, or Mars, or some of the
moons of Saturn and Jupiter. Methanodesulfokores washburnensis is an
ancient type of archaea that harbors genes for two of the earliest
metabolisms of life, and it lives in a hot spring that mimics the environmental
conditions thought to have been prevalent on the early Earth. What other
clues about the origins of life on Earth and the evolution of archaea might we
discover as we continue to study high-temperature systems in Yellowstone
National Park?