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The Theme of Evolution in Photosynthesis

Many believe that, early in the Earth's history, microorganisms consumed organic molecules in much the same way that most animals and microorganisms do today. As you might imagine, these ancient organisms ran into a little problem: they were eating away all of the available food but not producing any of their own. (Sounds a little bit familiar, doesn't it? Looks like humans still have a lot in common with microorganisms.) Even though ancient organisms hadn’t even seen a cake yet, they were still trying to have it and eat it, too.

Estimates suggest that photosynthetic organisms appeared on Earth about 3.5 billion years ago._CITATION_UUID_7F4C7C972DF444F58B7EEFE631D74342_ The original photosynthetic organisms may have actually used hydrogen sulfide (H2S) as their electron source. Scientists then speculated that cyanobacteria (named for their color) evolved the ability to split a water molecule (H2O), which made the super-strong reducing agents needed for fixing carbon dioxide (CO2) and producing the carbohydrates required for life as we know it. The evolution to water-driven photosynthesis must have required a lot of changes in how organisms at the time conducted photosynthesis. The main reason for the needed changes is that H2O holds onto its electrons a lot better than H2S does. Once organisms figured out how to pull H2O into the reaction, a lot of oxygen (O2) was pumped into the atmosphere, and organic materials began to accumulate on the young Earth.

While we might currently believe that the production of O2 was nothing but a fantastically wonderful step forward for life on Earth, O2 in the atmosphere actually created a problem for early life forms. O2 is a great oxidizing agent, meaning that it can pull electrons away from other biological compounds in the cell. The gradual increase in O2 in the atmosphere provided a means of selection. In other words, organisms had to evolve protective methods to prevent oxidative damage to their cells…or they would die. Many organisms eventually acquired the ability to live in the presence of O2. Other organisms shielded themselves from O2 in the environment by settling into environments where they were not exposed to O2. Need an example? The microbes that live in the human gut are one. Yummy.

When we talk about photosynthesis in today's world, we often think about plants and the chloroplast. You might be surprised to find out that the chloroplast was a rather late addition in evolutionary history. Scientists hypothesize that, more than a billion years ago, a eukaryotic cell engulfed a photosynthetic bacterium through a process called endosymbiosis. This "little" event may have resulted in the chloroplast organelle that we know and love so much. While present-day chloroplasts are the result of many, many years of evolution, some artifacts of the chloroplast's previous life as a separate and independent entity remain. Spooky.

Scientists think that both mitochondria and chloroplasts organelles were the results of endosymbiotic events. What led scientists to hypothesize the endosymbiotic theory? Mitochondria and chloroplasts both contain their own genomes (read: sets of encoded DNA), although many of the genes needed for photosynthesis and respiration have been incorporated into the bigger cell’s nuclear genome. Nonetheless, the genome that stayed behind in these organelles tells us a lot about their evolutionary history. Scientists found that many of these genes more closely resembled bacterial genes than equivalent genes in the bigger cell’s nuclear genome. Mitochondria and chloroplasts also encode for their own tRNAs (transfer ribonucleic acids), and in some cases, they even have a different genetic code! Therefore, occasionally, a codon, or a set of three adjacent nucleotides from the RNA molecule that usually represent one amino acid, might code for one amino acid, while the same codon derived from the chloroplast genome might encode for an entirely different amino acid. Crazy stuff. Mitochondria and chloroplasts also segregate their deoxyribonucleic acid (DNA) in a manner completely independent from the bigger cell's nucleus.

All in all, the photosynthetic organisms that we see today are the product of millions, or even billions, of years of evolution. Over time, organisms have made small changes to their existing photosynthetic machinery. If these changes happened to give an organism an advantage over other organisms, the changed organism would be more likely to survive. Therefore, these changed features were "selected" through survival of the "fitter" organisms. However, there is no "best" way to do photosynthesis. Photosynthetic organisms closely evolve and adapt to their own special environments, and the result is a diverse array of existing photosynthetic organisms. In fact, a trait that may have benefited one organism's survival in one environment may actually be detrimental to another organism's survival in a different environment. A very serious take on "whatever floats your boat."

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