Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. The origin of the eukaryotes--the kingdom of life that includes all of the higher plants and animals, including ourselves--took place in the heavily obscured early history of the earth. Consequently, there is still much speculation involved in answering this question.
Carl Woese, a professor of microbiology at the University of Illinois at Urbana-Champaign and the discoverer of archaebacteria, offers one reply: "Evidence from microfossils strongly suggests that life arose on the earth long ago, probably within a few hundred million years of the planet's formation.
Get smart. Sign Up. Support science journalism. In rocks that are 3. There are different forms of carbon called isotopes. Living things use a certain carbon isotope 12 C and exclude a second isotope 13 C while neither is discriminated against in the formation of minerals.
After more than 10, analyses of carbon ratios in rock sediments, there is only one period in which a qualitative difference is observed. At around 3. This analysis suggests that living things were present on earth 3.
This change in carbon isotopes occurred shortly after the appearance of liquid water on the planet Brocks, ; Schidlowski , ; Holland, Rocks dated at 3. In the rock strata of Isua , Greenland, there are "fossils" of small organic material which are 3. Although there is organic matter inside them and there are examples of them splitting budding in the way that yeast do , it is not clear whether they are living cells or nonliving microspheres Pflug , ; Roedder , ; Bridgwater , ; Dunlop, There have been " nanofossils" found on Martian rocks which contain organic material.
Some feel that these tiny structures have were formed by primitive organisms on Mars although many disagree with this interpretation Trevors , b. The biomineralization of ancient microbes which resulted in the formation of microfossils often is the result of the incorporation of metal ions on the cell surface. Nanobacteria, which range in size from 0. Most fossils are found in a type of rock known as sedimentary rock. The oldest known sedimentary rock deposit is the 3. Given that the oldest known rocks are only 4.
Although there are no fossils known in Isua , carbon ratios in this rock are similar to those found in later rocks which contain fossils of photosynthetic microbes McClendon, Very small carbonaceous microstructures are known in 3.
The oldest microfossils from Australia are 3. In the anammox bacteria mentioned above, there is also an additional but this time single-membrane-bounded organelle, the anammoxosome Figure 4 , containing at least one enzyme important for their unique anammox ammonia-oxidizing physiology in the absence of oxygen van Niftrik et al Another interesting feature of planctomycete cells is their condensed nucleoid, a mass of folded chromosomal DNA fibrils visible in sectioned cells via electron microscopy, contrasting with the nucleoids of other bacteria which tend to be spread throughout the cytoplasm.
In the case of Gemmata obscuriglobus this has been linked to the high resistance of cells in this species to UV and gamma radiation Lieber et al The planctomycetes are thus Bacteria that form a significant exception to simple prokaryote structure. Are there possibly other exceptions to be discovered? Why are planctomycetes important for understanding evolution of eukaryotes?
The other type of hypothesis involves "autogenous" formation of internal membranes from only one non-eukaryote cell type via an infolding of cytoplasmic membrane to form an endomembrane system and eventually to form a nucleus Cavalier-Smith ; Cavalier-Smith Nutrient acquisition may have selected for this de Duve We don't yet know to what extent compartments of planctomycetes can be explained by molecular mechanisms similar to those of eukaryotes.
Even if it turns out that there are no direct links between planctomycetes and eukaryotes i. Planctomycete cell structure differs from that of all other known prokaryotes. We are not sure yet whether the cell plan of a planctomycete like Gemmata resembles that of eukaryotes at a deeper molecular level.
Such a eukaryote-like cell plan occurring in a member of the Bacteria suggests that it may be worth looking for such deeper similarities. An interesting recent development suggesting that planctomycetes and their cells may possess deeper resemblances with eukaryotes is the discovery of an endocytosis-like process for protein uptake by cells of Gemmata obscuriglobus involving membrane infolding and vesicle formation Lonhienne et al Like the membrane-bounded nuclear body in this species, such a process is unknown in other Bacteria or Archaea.
This is consistent with the nutrient-acquisition-driven model for endomembrane evolution referred to above as a possible first step on the road to eukaryotes. Could planctomycetes have a whole series of further characteristics previously known only in eukaryotes, including eukaryote-homologous genes and proteins?
Cell biologists and microbiologists don't know yet, but are excited by this prospect which may lead to new insights into the evolutionary relationships between simpler and more complex kinds of cells. Cell biologists and microbiologists have in the past thought that only two major types of structural cell organization exist, prokaryotic — where the chromosomal DNA is naked and exposed within the cell cytoplasm — and eukaryotic — where chromosomal DNA is surrounded by a membranous envelope.
All members of domains Bacteria and Archaea have been thought to possess prokaryotic organization. The planctomycete bacteria however complicate this simple scheme, since their cells are compartmentalized by internal membranes including those surrounding the chromosomal DNA. Are they a third type of cell organization that originated independently of other known types, or are they key to understanding how eukaryotes and their complex internal structure evolved?
In either case they are significant for understanding the evolution of cells and the diverse ways in which cells can potentially evolve. Angert, E. The largest bacterium. Nature , — doi Cavalier-Smith, T. Concept of a bacterium still valid in prokaryote debate. Nature , doi The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa.
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Multiple secondary origins of the anaerobic lifestyle in eukaryotes. Fuerst, J. Intracellular compartmentation in planctomycetes. Annu Rev Microbiol 59 , — doi Membrane-bounded nucleoid in the eubacterium Gemmata obscuriglobus. The planctomycetes: emerging models for microbial ecology, evolution and cell biology.
Microbiology Pt 7 , — Glansdorff, N. The last universal common ancestor: emergence, constitution and genetic legacy of an elusive forerunner. Biol Direct 3 , 29 doi Kartal, B. Sewage treatment with anammox. Science , — doi Kuenen, J. Anammox bacteria: from discovery to application. Nat Rev Microbiol 6 , — doi Kuper, U. Energized outer membrane and spatial separation of metabolic processes in the hyperthermophilic Archaeon Ignicoccus hospitalis.
Lindsay, M. Cell compartmentalisation in planctomycetes: novel types of structural organisation for the bacterial cell. Arch Microbiol , — In the typical human body, prokaryotic cells outnumber human body cells by about ten to one.
They comprise the majority of living things in all ecosystems. Some prokaryotes thrive in environments that are inhospitable for most living things. Prokaryotes recycle nutrients —essential substances such as carbon and nitrogen —and they drive the evolution of new ecosystems, some of which are natural and others man-made.
Prokaryotes have been on Earth since long before multicellular life appeared. Indeed, eukaryotic cells are thought to be the descendants of ancient prokaryotic communities.
When and where did cellular life begin? What were the conditions on Earth when life began? We now know that prokaryotes were likely the first forms of cellular life on Earth, and they existed for billions of years before plants and animals appeared. The Earth and its moon are dated at about 4. This estimate is based on evidence from radiometric dating of meteorite material together with other substrate material from Earth and the moon.
Early Earth had a very different atmosphere contained less molecular oxygen than it does today and was subjected to strong solar radiation; thus, the first organisms probably would have flourished where they were more protected, such as in the deep ocean or far beneath the surface of the Earth. Strong volcanic activity was common on Earth at this time, so it is likely that these first organisms—the first prokaryotes—were adapted to very high temperatures.
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