Common Complaint: How did multicellular life originate? How did cells adapted to individual survival ‘learn’ to cooperate and specialize (including undergoing programmed cell death) to create complex plants and animals? [1]
Answer: Remember: Evolution does not mean the origin of life; it means changes in population over time. With that understood, we do have multiple lines of evidence of the evolution of multicellular life from single-cellular life. A famous example of the evolution of multicellular cooperation is the mitochondria. The mitochondria was once an independent cell, a type of bacteria, with its own DNA that had the ability to produce usable energy (turn sugar into cellular energy). The bacterium then was injested by another cell, which in cellular terms means engulfed in a membrane until enzymes could break it down. Inside the cell, the mitochondria could still produce energy, but that energy now directly benefited the cell. Instead of digesting the mitochondria, when mitosis occurred, both split into two, creating two new cells each with their own mitochondria. Cells that didn't digest the mitochondria were more successful than those that did, resulting in a greater portion of mitochondria-containing cells in the future. The presence of mitochondria is what has allowed eukaryotic organisms to be so successful. The chloroplasts in plants is another example of this cooperation (called endosymbiosis); they also were engulfed by bacteria that already had a mitochondria, and the cycle repeated.
We also have evidence of how cells could initially "group" into multicellular organisms. We've seen colonial behavior occur in the lab with cells that otherwise aren't multicellular (it's been documented at least 16 times), we've observed bacteria engage in cooperative swarming, we have single-celled protists that produce proteins that are used in intercellular interactions, and we've even seen multicellular "intermediate" steps like protists with multiple nuclei (that's a pretty close step to two nuclei in two adjacent cells). Because multicellular life is very beneficial (allows cells to specialize and work more efficiently), once present, it would be easily propagated in future generations.
As for programmed cell death, that's an active area of intense research. There are a lot of different hypotheses about its evolution including one called "the original sin" hypothesis, another one that looks at it over the course of 4 billion years, and still another that looks at it in terms of the necessary factors and how they evolved independently. And the truth is that science doesn't know. And that's okay. There are many things that science still has yet to discover and even study, and that's part of the beauty of science. "I don't know" is probably the most humble and most genuine phrase that can come from anyone or any field, and I think that that type of honesty should be applauded and encouraged, not ridiculed and condemned. The mere fact that science can honestly say, "I don't know" about things tells me that science can be trusted, for it is when we are afraid of the unknown that we replace honest uncertainty with dishonest affirmation. When we are knowingly dishonest in order to protect what we believe, then that brings the original belief into question. It begs us to ask, "What else is dishonest? What else is flawed?" It is in declaring false certainty that we bring about our downfall. If we must fill gaps of knowledge, both spiritual and physical, with our own limited excuses, then we are only ensuring its hastened destruction the same as we would by repairing a temple using cloth.
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