When biologists try to explain how life came to exist on Earth, they use the image of the “Tree of Life.” The tree metaphor appears in biblical Genesis, Kabbalah, the New Testament’s apocalyptic Book of Revelation, the Quran, and Buddhist, Hindu, and Mesoamerican literature and iconography.
Our origins represented in religion proved a useful tool for the emerging science of the Enlightenment with the image of a tree, sometimes with roots, and other times, a trunk and numerous branches. This became the linear model for the theory of how life came to be on Earth, starting with the first cell and branching out into the representational tree. The assumption is a single seed, cell, and point of origin.
Why do we look at origins when it comes to life in such a linear way?
What Predated Life?
A recently published article in Scientific American asks the question “How did life begin?” It describes the research of a team led by evolutionary biologist Bill Martin, from Heinrich Heine University Düsseldorf in Germany. They argue that evidence shows that life began twice on Earth.
The question of where life began is discussed in the article, pointing to hydrothermal vents as the most likely source of “genesis,” the name that inspired the field of genetics and derived from the Greek γÎνεσις, meaning “origin.”Â
Hydrothermal vents, Martin argues, are likely where our universal common ancestor emerged. Called the ur-organism, it was a bacterium-like cell from which all life on Earth descends. Was it alive? Not necessarily. It may have been more like a self-contained, membrane-sheathed droplet filled with chemical ingredients that were the precursors of life, including some form of genetic encoding.
The article talks about a half-alive ancestor that responded to the energy and mineral output of the hydrothermal vent environment that introduced prebiotic elements, the second origin factor, to the mix. The ur-organism’s absorption of the prebiotic element, when combined with its encoding capability, allowed, during replication, the passing along of genetic traits to create the first functioning cell. Martin calls it “one origin of the genetic code but two origins of life.”
Multiple Origins Abound
As interesting as the Martin team hypothesis is, others point to a multiple origins hypothesis.
Why would life emerge from a “singularity” when so much of what we observe on Earth today doesn’t reflect single origins?
For example, look at the emergence of human language. Today, humans communicate using over 7,000 spoken languages. Of these, a little more than half are written. Characterizing language has produced incredible discontinuities. Look at Egyptian hieroglyphics versus Greek. They bear no resemblance to each other even though their origins are in reasonable geographic proximity. When hieroglyphics became lost to history, it took a Rosetta Stone for us to decipher the former because of the accompanying Greek translation.
At a restaurant a few days ago, one wall was covered in Thai script. When I did some research, I found that the Thai written language originates from South India. Like hieroglyphics, it bears no resemblance to the written languages to which we are more familiar. The same can be said about Chinese, 3,300 years ago, or Mesoamerican scripts from Central Mexico and the Yucatán, 2,700 years ago.
Did an Ancestral Cell Lead to All Life?
What supports a single origin and the subsequent tree is the one thing we find common to all living things here on Earth: DNA (deoxyribonucleic acid).
Most scientists see DNA’s emergence as the genesis moment. Could other molecules have existed here on Earth to produce life based on a different set of rules? What if life in the Universe could have entirely different molecular origins, each begetting its own Tree of Life?
Today’s biology begins with the tree supporting multiple branches coming from common ancestors. And those representations show a trunk often supported by a bundle of roots; none of the latter represent an alternate molecular derivation.
Abiogenesis: A Different Genesis Story
The theory of alternative biochemistries with different molecules leading to life is called abiogenesis. Here on Earth, DNA triumphed, but what was here before that victory?
Self-replicating RNA (ribonucleic acid), the simpler nucleic cousin of DNA, may have ruled before it took the former’s architecture and bound it to sugars.
Return missions from asteroids have brought back samples containing the nucleic and amino acid components common to DNA and RNA: adenine, guanine, cytosine, thymine and uracil. These samples have also been infused with sugars and thousands of other life-friendly chemicals.
In a pre-RNA and DNA world, we could have seen TNA (threose nucleic acid), PNA (peptide nucleic acid) and GNA (glycol nucleic acid) self-replicating molecules.
DNA’s eventual triumph over the three others mentioned above, other competitors, and RNA may have resulted because it was chemically more complex, which gave it greater flexibility. DNA proved to be more stable during replication than RNA, with lower mutation rates. Then DNA developed a collegial relationship with RNA, an effective partnership that other complex molecules may not have been able to duplicate. It proved to be a winning combination based on what we see here on Earth today.
Is DNA the only molecule in the Universe capable of producing life? In laboratories, experiments have produced synthetic alternatives to DNA. Would DNA likely produce cellular life elsewhere? Changes to Earth’s environment, such as much hotter or colder temperatures, higher radiation exposure, and highly acidic or alkaline water, would have made DNA less likely as the originator of life on Earth.
