The legendary Grade 12 exam preparation is supposed to be a comprehensive and systematic review, but for us, it is not a review, but a learning process. I have found that many knowledge points were rarely encountered before, which undoubtedly points to two issues: my previous studies were quite superficial, and my power of forgetting was too strong. Take biology, for example; in the past, I always skipped through it simply and never thought deeply about certain issues. Now, in the current “re-review” phase, it has sparked many interesting reflections. In particular, the discussion regarding cellular evolution is exceptionally fascinating.
According to paleontological research, the first life on Earth originated 3.2 billion years ago as a very simple prokaryotic cell. Its genetic material was RNA, which later gradually evolved into DNA as the genetic material; for example, bacteria have a circular DNA molecule. Prokaryotes quickly evolved into eukaryotes, as the oldest known eukaryotic fossils to date are nearly 2.1 billion years old. Many scientists speculate that the earliest eukaryotes may have appeared as early as 3 billion years ago.
This leads to a question: when did viruses appear? How did they appear?
According to research, the complexity of prokaryotes lies between viruses and eukaryotes, yet prokaryotes originated the earliest. According to evolutionary theory, eukaryotes should have evolved from prokaryotes, but what about viruses? Where did this “monster,” composed only of genetic material (DNA or RNA) and a protein shell, come from? Did it “degenerate” from prokaryotes? Did prokaryotes at some point part ways, splitting into two “factions”—one following a complexity theory to become eukaryotes, and the other taking a simple route to become viruses? Checking Wikipedia, I found that some scientists indeed hold this view.
Next are reflections on eukaryotic cells. Eukaryotic cells have cytoplasm, a cell membrane, and a nucleus, as well as many organelles (ribosomes, mitochondria, etc.), and chromosomes within the nucleus. High school biology rarely explains the specific components of these substances in detail, but a little extension reveals many interesting things.
First are mitochondria and chloroplasts. Both organelles have membrane structures on the outside and contain a circular DNA molecule inside; in other words, they also contain certain genetic material. Furthermore, they have ribosomes and can synthesize proteins (such as enzymes for photosynthesis or respiration). Moreover, the diameter of mitochondria is generally 0.5–1.0 \mum and the length is 1.5–3.0 \mum, which is similar to the dimensions of prokaryotic cells. Or rather, they are essentially bacteria! Consequently, some have exclaimed: Is it possible that some ancient cells swallowed certain bacteria and formed a symbiotic relationship with them, subsequently evolving into the eukaryotic cells we see today with mitochondria and chloroplasts? If that is the case, is the essence of biological evolution not just a series of “Great Mergers”? (Wikipedia)
There are other interesting thoughts: ribosomes are composed of ribosomal RNA (rRNA) and proteins, and chromosomes are composed of DNA and proteins. Is this not similar to viruses? Their sizes are also similar to viruses. Does this allow us to guess that in the “Great Mergers” of the past, eukaryotic cells swallowed not only some bacteria, but also some viruses?
There is an even more interesting proposition: there were originally no viruses in the world, but the ribosomes in those cells could not stand the “tyranny” of their hosts and no longer wanted to be exploited, thus “rebelling” to “set up their own house,” eventually forming viruses?
I do not know; we are just musing...
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