Imagine traveling back through the timeline of life on Earth. You would pass the age of dinosaurs, the Cambrian explosion, and the emergence of multicellular organisms, all the way to a tiny speck in a primordial ocean approximately four billion years ago.
This speck is known as LUCA -- the Last Universal Common Ancestor. LUCA has long been considered the earliest organism we can trace through evolutionary family trees, representing a pivotal point in biological history.
However, LUCA was not entirely primitive.
When scientists analyze LUCA's genome using advanced bioinformatics and molecular phylogenetics, they uncover a fully functional cellular entity. This organism possessed a complex metabolism, a genetic code in DNA, and ribosomes for protein synthesis.
Biologists Aaron Goldman, Gregory Fournier, and Betül Kaçar emphasize in their recent perspective for Cell Genomics that the study of early evolutionary history reveals how the essential characteristics of life emerged.
If LUCA was already a sophisticated organism, it implies that the foundational story of life's origins -- the chaotic transition from non-living chemistry to biology -- occurred much earlier than LUCA's time.
Recent findings highlight a specific class of "fossil" genes embedded in our DNA, granting insights into this ancient epoch.
The Molecular Time Machine
To extend our understanding beyond LUCA, Goldman, Fournier, and Kaçar advocate for the exploration of "universal paralogs."
These are unique families of genes that duplicated prior to the existence of LUCA.
Typically, when constructing a gene family tree, the lineage halts at LUCA, leaving us without a reference to identify older branches.
Occasionally, a gene in a pre-LUCA organism would duplicate itself, resulting in two copies that evolved distinct functions. When LUCA emerged, it inherited both copies -- known as "paralogs."
Since these copies are present in nearly all living organisms today, from bacteria to humans, we can compare them to glean insights about their ancient origins.
Fournier, a geobiologist at MIT, states, "The history of these universal paralogs is our only window into the earliest cellular lineages, and we must extract as much knowledge from them as possible."
This process resembles analyzing two different versions of an ancient manuscript. By examining the variations and errors in both copies, researchers can reconstruct the original text that predates them.
Goldman, a biologist at Oberlin College, adds, "While LUCA is the most ancient organism we can study through evolutionary methods, many genes within its genome are significantly older."
Life Was a Generalist
What insights does this molecular time machine provide about the life forms that existed before LUCA?
In contemporary biology, proteins tend to specialize, performing specific tasks with great efficiency. However, the ancestors of these proteins were more versatile.
For instance, the cellular machinery responsible for translating genetic code into proteins today employs two distinct proteins, IF2 and EF-Tu, each with specialized roles. However, the pre-LUCA ancestor of these proteins was likely a generalist, capable of initiating and extending protein synthesis.
A similar trend is observed in enzymes that attach amino acids to tRNA molecules, a crucial step in converting genetic code into physical matter.
In the pre-LUCA world, the enzyme responsible for handling several amino acids did not differentiate between them, showcasing a more flexible approach to survival.
The Dawn of Cellularity
A significant question in evolutionary biology is whether life originated within cells or as a mixture of chemicals. Universal paralogs are helping to clarify this debate.
These known universal paralogs are not random; they cluster around three essential survival functions: protein synthesis, amino acid processing, and transporting materials across membranes.
For example, the Signal Recognition Particle (SRP), which transports proteins to the cell membrane, had a pre-LUCA ancestor already adept at membrane targeting and receptor functions.
The presence of these ancient membrane transporters indicates that organisms before LUCA were indeed cellular entities. "Proteins involved in membrane functions suggest that cellularity existed at this time," the authors assert.
Resurrecting the Past with AI
Goldman's team has utilized these genetic maps to reconstruct ancient proteins in the laboratory. They synthesized a pre-LUCA protein involved in membrane insertion, demonstrating its ability to interact with modern protein synthesis machinery.
Kaçar, a bacteriologist at the University of Wisconsin-Madison, notes, "By studying universal paralogs, we connect the earliest stages of life on Earth to contemporary scientific tools, transforming the deepest unknowns of evolution into testable discoveries."
The future of this field appears promising. The authors highlight advancements in deep learning, such as tools that accurately predict protein structures, as transformative.
Computers can model how these ancient proteins may have folded and functioned billions of years ago. Coupled with laboratory experiments to resurrect ancient proteins, we are progressing from merely inferring the past to actively reconstructing it.
