The question of how life first arose on our planet is one we have not yet fully answered, but science is constantly getting closer - and a new study identifies the structures of the proteins that may well have made it happen. .
To begin with, the team behind the study decided to take as a starting point that life as we know it depends on collecting and using energy. In the primordial soup of ancient Earth, this energy would most likely have come from the sky, in the form of radiation from the Sun or from deep inside the Earth itself, like heat seeping through hydrothermal vents at the bottom of the ancient seas.
At a molecular level, this energy consumption means the transfer of electrons, the basic chemical process that involves an electron moving from one atom or molecule to another. Electron transfer is the very core of oxidation-reduction reactions (also known as redox reactions), which are crucial to some of life's basic functions.
Since metals are the best elements for performing electron transfer, and the complex molecules called proteins are what drive most biological processes, the researchers decided to combine the two and search for proteins that bind metals.
A methodical, computational approach was used to compare metal-finding proteins, revealing certain common features that matched across them all - regardless of the protein functionality, the metal it binds to, or the organism involved.
"We saw that the metal-binding nuclei in existing proteins are actually similar, although the proteins themselves may not be," says microbiologist Yana Bromberg of Rutgers University-New Brunswick in New Jersey.
"We also saw that these metal-binding nuclei often consist of repetitive substructures, a bit like Lego bricks. Oddly enough, these blocks were also found in other areas of the proteins, not just metal-binding nuclei, and in many other proteins that were not taken into account in our examination."
These common features may well have been present and active in the earliest proteins, the researchers suggest, changing over time to become the proteins we see today - but retaining certain common structures.
The idea is that soluble metals in the archaic ocean that covered the Earth thousands of millions of years ago could have been used to drive the electron mixture required for energy transfer and in turn biological life.
"Our observation suggests that rearrangements of these small building blocks may have had a single or a small number of common ancestors and given rise to the whole range of proteins and their functions that are currently available," says Bromberg. "That is, to life as we know it."
In particular, the team was able to identify developments in protein folds - the forms that proteins adopt as they become biologically active - that may have produced the proteins we know today, almost like a molecular pedigree project.
The study also concludes that biologically functional peptides, the smaller versions of proteins, may have preceded the earliest proteins, which go back as far as 3.8 billion years ago. All of this contributes to our understanding of how life first started.
As always, any analysis of the beginning of life on Earth can also be important in looking for life on other planets, where life may begin to evolve (or may have already evolved) along similar biological paths.
"We have very little information about how life originated on this planet, and our work contributes with a previously inaccessible explanation," says Bromberg. "This explanation could also potentially contribute to our search for life on other planets and planetary bodies.
"Our findings of the specific structural building blocks may also be relevant to synthetic biology efforts, where researchers aim to reconstruct specifically active proteins."
The research is published in The progress of science.