Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science. 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. Knowledge awaits. The development from an egg into an adult is somewhat similar on a much smaller scale and in an infinitesimal fraction of time to the evolution of the prokaryotic clone into a global superorganism.
Each of these two types of clones is therefore using its own genes very differently to give rise to specialized cells: in prokaryotes, this is the result of having different evolutions and adapting to new situations by gene exchanges and various cell associations. The global prokaryotic superorganism presumably reached its own type of adulthood about two B. Its stability since the appearance of the eukaryotes is a surprising evolutionary fact.
Instead of being taken over by the other, both eukaryotes and prokaryotes collaborated in many instances and this seems to have insured a longer and more successful life for both groups.
This evolution of prokaryotes, however, was seriously limited in one way: it could not produce three-dimensional multicellular organisms with compact tissues. All their evolution had been in the direction of the smallest possible disjunct cells with a specialized and limited bio-energetic activity. They tended to remain physically separated in liquid or viscous surroundings. It is likely that for, possibly, the first 1 B Y.
Later, one innovative evolutionary episode seems to have given birth to larger cells predators on the more conventional type of prokaryotes. Surprisingly, these presumed predators do not seem to have had a profound impact as rivals or ennemies of the other prokaryotic type. Their number in natural habitats does not appear high nor significant.
Predators are believed, however, to have participated in a fateful evolutionary event. Being the First prokaryotic large cells, they could accept and accomodate smaller, cooperating symbionts; this opened the door for a major transition, a type of evolution which had not been accessible to their ancestors. Eukaryotes also seem to have originated from at least three different types of prokaryotic cells Margulis, which, when united together, formed an entirely new type of cell.
It also represented the first and possibly the last successful permanent endosymbiosis exclusively involving prokaryotic cells Margulis, The large cell which participated in this endosymbiosis was probably an Archaea and a predator Guerrero et al, ; de Duve, One often overlooked aspect of this major biological event is that each of these permanently joined symbionts had a long uninterrupted past of participation as a member of close associations of strains in dynamic prokaryotic communities.
Their long adaptation to life together in different, successful associations made them good candidates for putting together compatible groups of genes which contributed to the success of the earliest eukaryotes. Organisation of the nucleus followed later, starting with the fusion of the large replicon of the predator cell with that of a spirochete and the inclusion of their small replicons Sonea, Once the nucleus and its membrane were later completed, early eukaryotic cells had enough intracellular genes of their own and no immediate need for visiting small replicons.
Newly independent eukaryotic cells became full unicellular organisms whose offspring could evolve in many directions without being restricted by the rules of team life which applied to their physically separated prokaryotic ancestors. Eukaryotes no longer had to collaborate with the global prokaryotic superorganism. In eukaryotes, the genetic isolation resulting from the early loss of small replicons and of the mechanism of transformation has been compensated for, but only in part, by the capacity to synthesize entirely new genes by serial random mutations which, probably, had lost importance in prokaryotes approximately 2 B.
As already stated, the latter is absent in prokaryotes. For hundreds of millions years eukaryotes conceivably remained unicellular and dependent on prokaryotes for food Sonea, Later, one very important but often overlooked source of new genes for the developing eukaryotic world appeared. It consisted of highly innovative symbioses with complementary types of prokaryotic cells.
This happened much later, as a large number of prokaryotes joined, as symbionts, different eukaryotes, starting with the protoctists, continuing with the ancestors of animals and, later, those of plants Margulis and Fester, Such associations became easier for the eukaryotic cells, which could often integrate the future symbionts into their increasingly larger cells. They participated successively in the origin of new, essential photosynthesizing entities by association with protoctists, thus producing many new and successful forms of life, the best known being the unicellular eukaryotic green algae which populate oceans and lakes.
Much later, multicellular algae, lichens and plants were the result of symbioses between cyanobacteria and multicellular eukaryotes fungi Honneger, These new associations made life possible and abundant under sunlight at different levels in the ocean or on the surface of soil or rocks of the continents. Multicellular, tri-dimentional and large beings began to populate the earth. Bryophytes and tracheophytes produced branching roots that increase access to water and minerals from below the surface of the soils.
These are carried through conduits to vertical stems. This plant tissue supports the exposure to sunlight of fronds, leaves or needles containing chloroplasts undisputable descendants of former cyanobacteria. Over the same area, plants expose to sunlight a much larger surface than the previous, thin prokaryotic cover on the soil. Increasingly complex plants prospered and around million years ago they covered much of the continents with abundant growth. Benefiting from this rich new biomass, soil bacteria and land-dwelling animal species also evolved.
This put an end to the three billion years of practically lifeless history of the continents. Thus, a most fateful evolutionary innovation on the continents, and a giant evolutionary transition were the extension of photosynthetic cyanobacteria as symbionts to the third dimension by multicellular tissue.
After this most important event in the development of energy capture and transfer, the symbiosis between nitrogen fixing bacteria e. Azobacter aerogenes and many types of plants, particularly the legumes, also contributed to increase the biomass on the continents. A large number of animal species also need prokaryotic symbionts mostly ecto to survive in oceans or on continents Margulis and Fester, As already seen, the ruminants cattle, sheep, deer, camels, etc.
Termites use a similar arrangement and many insects and marine animals also depend on the metabolic versatility of symbiotic prokaryotes. A large proportion of all other animals which have no such special capacities are dependent on prokaryotic teams in their alimentary tract, without which they cannot live normally.
Experimentally obtained germfree animals may present serious often deadly health problems during their abnormal life. 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.
Because early Earth was prone to geological upheaval and volcanic eruption, and was subject to bombardment by mutagenic radiation from the sun, the first organisms were prokaryotes that must have withstood these harsh conditions. Microbial mats or large biofilms may represent the earliest forms of prokaryotic life on Earth; there is fossil evidence of their presence starting about 3.
A microbial mat is a multi-layered sheet of prokaryotes Figure 1 that includes mostly bacteria, but also archaeans. Microbial mats are only a few centimeters thick, and they typically grow where different types of materials interface, mostly on moist surfaces. The various types of prokaryotes that comprise them carry out different metabolic pathways, and that is the reason for their various colors.
Prokaryotes in a microbial mat are held together by a glue-like sticky substance that they secrete called extracellular matrix. Also found in the Pilbara region are fossilised remains of stromatolites. These are also mat-like structures of microbes that live in shallow marine environments and are still around today. Sand accumulates on top of the microbial mats, and the microbes move up towards the surface to get to the light again, making distinctive bulbous-shaped layers that eventually solidify into rocks.
Although we know that some living things thrive in more extreme conditions, the combination of warmth and water seem to be the most likely requirements for creating an environment that can support some kind of life—at least, the kinds of life forms similar to what we find on Earth. But who knows what other kinds of living things might exist? The origins of life on Earth Everything we know about life comes from a sample size of one: life here on Earth.
The streaky artwork of masses of cyanobacteria blue-green algae.
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