Who were the first producers on Earth, the last universal common ancestor (luca), the first autotrophs? The ancestor who, based on the fossil and nucleic acid sequence record (Woese), radiated out some 3.8 BYA and since then has left an unbroken chain of life.
Modern day autotrophs (bacteria, plants) produce complex organic compounds (fats, carbohydrates, proteins) from simple inorganic molecules (i.e. water, carbon dioxide) using chemosynthesis or photosynthesis. For example, plants drive the reaction shown below in the forward direction. This converts physical energy from the sun (photons) into chemical energy that becomes stored in the bonds of reduced carbon carbohydrates.
energy + CO2 + H2O <--> O2 + carbohydrates
By contrast, heterotrophs (animals and fungi) are consumers. We heterotrophs drive the reverse reaction, burning carbohydrates, to provide energy while producing carbon dioxide.
Yet when we wonder how the atomic thread once weaved through fabric from nonliving to living systems, some of us at Deerfield might be influenced by our distinguished alum, Professor Woese. Woese's work might be understood by first considering the functional diversity of life today. As mammals, we live by eating other life forms – plant, animal or both. The maples on campus live by fixing carbon dioxide. Microbes that cover nearly every DA surface live by metabolizing nitrogen, methane, sulfur, metals or hydrogen. Citing these couple examples it’s easy to remember life’s extreme diversity. To outwardly express diversity, life today contains variety in our genomes for the unique functions that are common to different organisms. Although life is polymorphic and diverse, it also contains phenomenal similarity. An example of life’s similarity is the molecule that Woese used to map the tree of life called ribosomal ribonucleic acid (rRNA). Nearly unchanged for billions of years, rRNA is so highly conserved because it functions the same in all types of organisms. That is, rRNA is part of the intercellular assembly line on which proteins are manufactured.
As depicted in the image below, it is the highly conserved ribosomes (a ribosome is structurally assembled from rRNA and protein), that functions in protein synthesis (thus under selective pressure to conserve it's sequences) that have made rRNA useful for phylogenic analysis.
Woese concluded that greater differences in rRNA sequence correlates with more distant relations. From his analysis of different organisms, he proposed the three domains of life: archaea, bacteria, and eukaryote. Furthermore, he suggests that on the basis of rRNA gene differences, the three domains of life arose separately from the last universal common ancestor, or common root of the tree of life.
Beyond suggesting that a universal common ancestor existed, some research presently focuses on understanding how such ancestral organisms may have metabolized, replicated and evolved.
Wachtershauser and others have proposed an iron-sulfur world theory, where reaction cascades and catalytic feedbacks (metabolism) drive an early beginning. This iron-sulfur world suggests a pyruvate metabolism where pyruvic acid is synthesized from formic acid in reducing environment. Such a cascade system is seen as attractive, since modern day citric acid cycle, amino acids and sugars employ similar pyruvate based pathways.
Szostak has taken a different approach in the study of common ancestors and early life. By building primitive cells, or protocells, that consist of two main components, a self-replicating genetic polymer and a self-replicating membrane boundary, they look for evidence that they systems will begin to evolve in a Darwinian fashion.
Who were the first producers on Earth, the last universal common ancestor (luca), the first autotrophs? The ancestor who, based on the fossil and nucleic acid sequence record (Woese), radiated out some 3.8 BYA and since then has left an unbroken chain of life.
