Since December, there have been tens of thousands of COVID-19 cases both in and out of China, caused by a previously unknown virus: novel coronavirus (新冠病毒).
It's not the first time for viruses to make headlines. Viruses have brought big trouble to humans, with their potential to cause widespread disease and death.
What are they? Viruses are considered to be non-living organisms (有机物). They can infect animals and plants, making them sick. They contain genetic materials like DNA and are protected by a coating of protein.
Unlike bacteria (细菌), viruses can't reproduce on their own. Instead, they invaded the cells of living organisms to reproduce, spread and take over. But the basic question is: where did viruses first come from?
Until now, no clear explanation for their origin exists. "Tracing the origins of viruses is difficult," Ed Rybicki, a virologist at the university of Cape Town in South Africa, toldScientific American, "because viruses don't leave fossils (化石) and because of the tricks they use to make copies of themselves within the cells they've invaded. "
Understanding the origins of viruses means fully understanding the history of their hosts—not only humans, but also bacteria, plants and other animals, which is a complicated task, saysNature.
However, there are three main theories to explain the origin of viruses. First, viruses started as independent organisms, and then became parasites (寄生者). Second, viruses evolved (进化) from pieces of DNA or RNA that "escaped" from larger organisms. The third one is that viruses co-evolved with their host cells, which means they existed alongside these cells.
For the time being, these are only theories. The technology and proofs we have today cannot be used to test these theories and discover the mostplausibleexplanation. Continuing studies may provide us with clearer answers. Or future studies may find that the answer is even more mysterious than it now appears.
