Geologists have estimated our Earth to be around 4.6 billion years old. Paleontologists have discovered fossilized remains of prokaryotic cells aged around 3.5-3.8 billion years in stromatolites and sedimentary rocks. What are Stromatolites? They are layered rocks, or in more technical terms stratified rocks, often rounded, that are formed by incorporation of mineral sediments (calcium sulphates, calcium carbonates) in microbial mats.
Fig: Stromatolites at Shark Bay, Western Australia
Modern day stromatolites are formed by cyanobacteria. Thus prokaryotic life arose at a slow pace after the earth’s surface cooled. It is thus apparent that the earliest prokaryotes were anaerobic.
Cyanobacteria and oxygen-producing photosynthetic mechanisms developed later, at around 2- 3 billion years ago. As the amount of oxygen in Earth’s atmosphere started increasing, microbial diversity also increased greatly. Carl Woese, a University of Illinois at Urbana-Champaign microbiologist and biophysicist, along with his collaborators, studied rRNA sequences in prokaryotic cells and suggested that prokaryotes divided into two distinct groups very early on.
Fig: The Universal Phylogenetic Tree, which depicts Woese’s views. The tree is divided into three distinct branches, which are the primary groups- Bacteria, Archaea and Eukarya (Eucarya)
The Archaea and bacteria first diverged, then eukaryotes developed. These primary groups collectively are called Domains and are placed above the Phylum and Kingdom levels. The domains are distinctively different from one another. Eukaryotic organisms with primarily glycerol fatty acyl diester membrane lipids and eukaryotic rRNA belong to Eukarya (or Eucarya). On the other hand, the domain Bacteria contains prokaryotic cells with bacterial rRNA and membrane lipids that are primary diacyl glycerol diesters. And the third domain Archaea is composed of isoprenoid glycerol diether or digycerol tetraether lipids in their membranes and archaeal rRNA. It likely appears that modern eukaryotes arose from prokaryotes about 1.4-1.6 billion years ago. How this occurred is not yet known till now, but two hypotheses have been proposed. The first theory says that nuclei, mitochondria, and chloroplasts arose by invagination of the plasma membrane to form double-membrane structures containing genetic material and capable of further development and specialization. According to the more popular endosymbiosis theory proposed by Lynn Margulis, the first event that occurred was nucleus formation in the pro-eukaryotic cell. This ancestral eukaryotic cell may have developed from a fusion of ancient bacteria and Archaea, possibly a gram-negative bacterial host cell that had lost its cell wall engulfed an archaeon to form an endosymbiotic association. The archaeon subsequently lost its wall and plasma membrane, while the host bacterium developed membrane infolds. Eventually the host genome got transferred to the original archaeon, and a nucleus and endoplasmic reticulum was formed. Both bacterial and archaeal genes could be lost during formation of the eukaryotic genome. At this point, it should be noted that most believe that the Archaea and Eukarya are more closely related than this hypothetical scenario implies. They propose that the eukaryotic liner diverged from the Archaea and then the nucleus formed, possibly from the Golgi apparatus. It appears that mitochondria and chloroplasts developed later. The free-living, fermenting ancestral eukaryote with its nucleus established a permanent symbiotic relationship with photosynthetic bacteria, which then evolved into chloroplasts. Cyanobacteria have been considered the most likely ancestors of chloroplasts. Mitochondria arose from an endosymbiotic relationship between the free-living primitive eukaryote and bacteria with aerobic respiration. Some have proposed that aerobic respiration actually arose before oxygenic photosynthesis and made use of small amounts of oxygen available at this early stage of planetary development. The exact development sequence is still not clear.
The endosymbiotic theory has received support from the discovery of an endosymbiotic cyanobacterium that inhibits the biflagellate protist Cyanophora paradoxa and acts as its chloroplast. The endosymbiont called cyanelle resembles the cyanaobacteria in its photosynthetic pigment system and fine structure and is surrounded by a peptidoglycan layer. It differs from cyanobacteria in lacking the
lipopolysaccharide (LPS) outer membrane characteristic of gram-negative bacteria. The cyanelle may be a recently established endosymbiont that is evolving into a chloroplast. Further support is provided by rRNA trees, which locate chloroplast RNA within the cyanobacteria. As of now, both hypotheses have supporters. Latest information and new data may help resolve the issue to everyone’s satisfaction. However these hypotheses concern that occurred in the distant past and cannot be directly observed. Thus a complete consensus on the matter might never be reached.
Written by Udeepta Phukan
Undergraduate Student, Department of Chemistry.
St. Xavier's College, Mumbai, India.