How did the earliest living forms look like? Scientists provide a method
for addressing this query in a recent perspective paper by looking at the
earliest evolution of electron transport chains, a universal metabolic
approach with a very old history. The article appears in the Proceedings of
the National Academy of Sciences publication.
The origin of life is still one of the biggest scientific mysteries despite
decades of research. According to Aaron Goldman, associate professor of
biology at Oberlin College, "the most fundamental characteristics of
biology—that organisms are made of cells, that they transmit genetic
information through DNA, and that they use protein enzymes to run their
metabolism—all emerged through specific processes in very early evolutionary
history."
Understanding how these most fundamental biological processes came into
being will help us better understand not just how life functions at its most
basic level but also what life is and how we could search for it outside of
Earth.
Laboratory experiments that mimic early Earth settings are frequently used
to investigate the origin of life. These experiments search for chemistries
that can produce the same sorts of biomolecules and metabolic processes as
those found in living things today. This method is referred regarded as
"bottom-up" because it uses components that would have existed on the
primordial Earth. These "prebiotic chemistry" tests have effectively shown
how life "may have" begun, but they are unable to explain how life "did"
originate.
While other study reconstructs the possible appearance of early life forms
using information about life today, using methods from evolutionary biology.
We can learn about the evolution of life on Earth by using this "top-down"
strategy.
However, top-down research can only go back as far as the genes that are
still present in living things today, not all the way to the beginning of
life. Top-down and bottom-up studies both attempt to understand the
beginnings of life, despite their differences, and ideally their conclusions
should point to a similar set of circumstances.
This methodological gap is addressed in a new work by Goldman, Laurie Barge
(Research Scientist in Astrobiology at NASA's Jet Propulsion Laboratory
(JPL)), and colleagues. The authors contend that it is possible to determine
how life actually did begin on the early Earth by combining bottom-up
laboratory studies on likely paths toward a beginning of life with top-down
evolutionary reconstructions of early life forms.
The authors of the paper, "Electron Transport Chains as a Window into the
Earliest Stages of Evolution," discuss an aspect of modern life that may be
examined by combining top-down and bottom-up research: electron transport
chains.
From bacteria to humans, species from all branches of the tree of life
employ electron transport chains as a sort of metabolic system to generate
useful chemical energy. For example, our mitochondria contain an electron
transport chain linked to our heterotrophic (food-consuming) energy
metabolism, whereas plants have a completely different electron transport
chain linked to photosynthesis (the generation of energy from sunlight). The
numerous types of electron transport chains are specialized to each form of
life and the energy metabolism they use.
Organisms utilize a wide spectrum of electron transport chains connected to
a number of various energy metabolisms across the microbiological world. The
authors propose multiple models for ancestor electron transport chains that
might go back to very early evolutionary history despite these
discrepancies, and they detail evidence from top-down studies indicating
this type of metabolic approach was utilized by the very oldest living
forms. Additionally, they review recent bottom-up data that suggests that
minerals and early Earth ocean water may have helped to allow electron
transport chain-like chemistry even before the origin of life as we know
it.
The authors describe prospective research techniques that combine top-down
and bottom-up study on the earliest history of electron transport chains in
order to better comprehend ancient energy metabolism and the genesis of life
in general. These tactics are motivated by these discoveries.
This study is the result of five years of prior work by the
multi-institute, multidisciplinary team lead by Barge at JPL to investigate
the possibility of metabolic processes developing in early Earth's
geological environments.
The team has previously studied topics such as specific electron transport
chain reactions driven by minerals (under the direction of Jessica Weber, a
JPL Research Scientist), how prebiotic chemistry may have been incorporated
into the active sites of ancient enzymes (under the direction of Goldman),
and microbial metabolism in severely energy-constrained environments (under
the direction of Doug LaRowe, of the University of Southern
California).
Barge asserts that in order to properly understand the emergence of
metabolism, a multidisciplinary team is required. "Our work has combined
top-down and bottom-up approaches using techniques from chemistry, geology,
biology, and computational modeling, and this kind of collaboration will be
important for future studies of prebiotic metabolic pathways," the authors
write.
Provided by
Oberlin College
