Tuesday, April 5, 2011
Do We Got Time?
On page 121, Shubin describes how much time the first organisms had to evolve into different multicellular organisms which soon turned into today's organisms. Many landmark events occurred to stimulate the "production" of multicellular organisms. Using outside resources, discuss which events in the history of the Earth led to the evolution of unicellular to multicellular organisms. Why would these certain events cause this change? Did these events change living conditions? Why would the change from unicellular to multicellular organisms be an advantage in evolution after these events? Shreeraj Patel shreeraj.patel1@gmail.com
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The earliest inhabitants of earth were single-celled organisms. Fossilized stromatolies tell us that the first documented sign of life goes back 3.5 billion years. They contain evidence of prokaryotes, which were the first inhabitants of the earth. Prokaryotes are the only inhabitants on earth because there is not enough oxygen to support more complex forms of life. Meanwhile, the oxygen was having a negative effect on the prokaryotes “attacking chemical bonds and inhibiting enzymes, damaging cells…doom[ing] many prokaryotic groups” (Campbell 516). When the amount of oxygen in the earth begins to increase, we then start to see eukaryotic cells forming, one step more complex than the prokaryotes, as we see cellular respiration being utilized and specialized structures such as mitochondria and other internal structures. This brings us to 2.1 billion years ago, where we see evidence of the first eukaryotic organisms. (Campbell 514-517)
ReplyDeleteWith these more complex eukaryotic organisms, the evolution of more diverse organisms was made possible. Most unicellular organisms then began to organize into colonies, one of the first steps in the evolution of multicellular organisms. In the Volvox for example, cytoplasmic bridges connect the cells in the colony, uniting the cells. The Volvox begins to show some division of labor that we see in multicellular organisms. But the disadvantage of unicellular organisms in colonies is that they are so closely tied, that they depend on each other and cannot survive on their own. (http://www.ncbi.nlm.nih.gov/books/NBK28332/)
Many hypotheses have arisen to answer the question of how exactly unicellular organisms evolved into multicellular organisms. One hypothesis, as is evident in the Volvox, is that the daughter cells did not fully separate, resulting in a colony that eventually developed specialized tissues. (http://www.nytimes.com/2010/12/14/science/14creatures.html) By making the transition from unicellular organisms to multicellular organisms, organisms were able to increase their size, and divide up their tasks with specialized cells, allowing them to become more efficient.
Another problem unicellular organisms faced was the diffusion of reactants. Due to their small volume, the rate of diffusion is slowed significantly, estimated to be more than 26 minutes. This greatly reduced the efficiency of the cell, as simple diffusion of necessary enzymes took such a long time. (Evolutionary biology, 2nd edition by J. Flegr). Thus, these evolved new multicellular organisms were given a selective advantage, as their cell processes took less time.
Anna leng (annaissbananas@yahoo.com)
There are many theories behind the reason why multicellular organisms started to develop. The Symbiotic Theory, also called the Endosymbiotic theory, was proposed by Lynn Margulis in the 1960s and states that eukaryotic organisms were derived from the joining of two or more prokaryote species. For example, if an oxygen breathing bacteria invaded an anaerobic amoebalike bacteria cell, they may perform mutually benefitting functions where “the bacteria would breathe for the anareobic amoebalike bacteria, and the amoebalike bacteria would navigate through new oxygen-rich waters in search of food” (http://facstaff.gpc.edu/~pgore/students/w96/joshbond/symb.htm). An event like this would only occur after a sufficient amount of oxygen had accumulated on Earth. This theory states that it is no coincidence “that oxygen begins to accumulate between the first fossil records of Prokaryotes and Eukaryotes” (http://facstaff.gpc.edu/~pgore/students/w96/joshbond/symb.htm). However, these first eukaryotic species were still only unicellular organisms. Margulis explains in her book Symbiosis in Cell Evolution (1981) that sometimes single celled organisms form a symbiotic relationship and become so dependent on each other that they cannot survive without each other. These two organisms will eventually incorporate both their genomes into one multicellular organism.
ReplyDeleteA second theory by Haeckel was formed in 1874, called the Colonial Theory. This theory claims that symbiosis of the same species led to multicellular organisms, unlike the Symbiotic Theory which associates two or more species. This theory explains two ways to bring about multicellularity: either “a single cell divides and its offspring stick together” or “several solitary cells aggregate to form a colony” (http://parts.mit.edu/igem07/index.php/Paris/Project_Description/Paris/colonial_theory_of_evolution_of_multicellularity). The first example of forming multicellular organisms occurs more in aquatic environments while the latter takes place in terrestrial environments. Several solitary cells aggregating are apparent in the creation of both Volvox and Gonium.
Multicellularity is a selective advantage in survival because it can create larger organisms, which are less likely to be consumed by predators. Larger animals also have a tendency to win when competing for resources. Also, with multiple cells doing different jobs, they can work together for the well being of the organism as a whole. In more complex multicellular organisms, tissues and organs become specialized for a certain function, so they don’t need to focus on other processes. Furthermore, multicellular species can communicate with each other, allowing all the cells in the organism to act as a whole. Lastly, multicellularity can lead to advanced methods of reproduction and protection of offspring during development, such as eggs and internal development. This helps with the cycle of surviving and reproducing.
Sources:
http://facstaff.gpc.edu/~pgore/students/w96/joshbond/symb.htm
Symbiosis in Cell Evolution by Lynn Margulis
http://parts.mit.edu/igem07/index.php/Paris/Project_Description/Paris/colonial_theory_of_evolution_of_multicellularity
http://silicasecchidisk.conncoll.edu/LucidKeys/Carolina_Key/html/Gonium_Main.html
http://www.botany.hawaii.edu/faculty/webb/bot311/bot311-00/celltissorgan/CellTissOrgan-2.htm
Austin Lee (austinklee7@gmail.com)
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ReplyDeleteFocusing on the later discussion of Austin Lee's post regarding advantage of multicellularity, one is able to further explore the structures of organisms and its developement throughout evolutionary history from a single celled organisms to complex mammals. One advantage of complex organisms that has not been discussed is the range of environments that complex organisms can live in.(provided that condition is not harsh enough to wipe out the entire population)
ReplyDeleteInitially, singled celled organisms under Kingdom Bacteria and Kingdom Archaea used concentration gradients inorder to overcome the osmotic pressure. One method was to regulate the salt concentration in their cytoplasma based on the salt concentration in the surrounding environment. Another method is named Organic-Osmolyte mechanisms where the cell builds up uncharged, highly water soluble compounds to maintain osmotic equilibrium. (http://www.publish.csiro.au/paper/EN06016.htm) In the case of bacteria and early life forms, their method of osmotic regulation involved passive diffusion by establishing concentration gradients as opposed to active transport.
With the advent of protists, came specialized organs in cellular level. With the developement of the nucleus and specifically the contractile vacuole in protists such as paramecium and amoeba, the cell could now actively pump out the water to osmoregulate. These specialized structures allowed more complex organisms to develope as they could now efficiently use ATP as opposed to bacteria that does not have any specialized organs.
The fungi and plants further developes the functions of specialized organs with the cell wall and the central vacuole. These structure allows even more range of environment that the organism can tolerate because they have the ability to storre water as well as establishing gradients to take in more water. This complex developement of multicellularity leads to even more specialized organs such as the xylem and the phloem water xylem transports water while the phloem trnasports food. However, one important information to note is that although protists had an active pump, with the advent of plants or kingdom Plantae, organisms as a whole have seemed to reverted back to the idea of diffusion and osmosis in osmoregulating its bodies. For example, trees undergo transpiration where the water is evaporated from the leaves and thereby creating a lower water potential at the top of the tree. This allows the movement of water upwards against the gravity. These adaptation were possible because of the complex multicelluar organisms with specialized organs developed. (Campbell)
Finally, within the Kingdom Animalia, multiple adaptations allow osmoregulation. First, while having many of similar methods such as concentration gradient, others involve sweating or urinating. Also, the developement from ectotherm to endotherm also adds to the advantage of complex organisms or rather causes it. Endotherms have the ability to regulate their internal temperature thus able to better regulate the biological processes in their body despite the harsh environment thus increasing the range of environment in which they can live in. For example, human kidney through process of diffusion of and osmosis is able to fine tune the osmoregulation in our body. (Campbell) Ectotherms, however, must stay near a water source in order to better control their internal temperature by either cooling it in the water and heating it in the sun. Multicellular organisms in all are advantageous because their complex system and specialized organs for regulating their catabolic and anabolic processes are better developed as opposed to bacteria or other primitive one-celled organism.
Kevin Jeon
bboybyung@gmail.com