A lot had been happening in the previous decade when 20,000 Leagues Under the Sea was first published in 1870.
In the previous decade, there had been a lot of warfare. There wars between China and the U.K., the U.K. and the Maori inhabitants of New Zealand, and between Imperial China and a large group of rebels numbering near half a million, who formed their own kingdom, before getting conquered by Imperial China, the war ending near the time of publishing. The American Civil war had also ended just a few years ago.
As for biology, only ten years earlier, Darwin’s Origin of Species had first entered print. The previous year, the first transatlantic cable had been laid. While dredging the ocean floor to make the channel for the long cable, zoologists discovered countless new flora and fauna. Gregor, a monk, after conducting a series of experiments, with plants, formulated the theories of “Mendelian Inheritance”.
The Telegraph union was founded in 1865, in Germany, Alfred Nobel invented an explosive, dynamite. The first transcontinental railroad was completed in the U.S. at Promontory Summit, Utah, in 1869. Whale harpooners were still in business at the time as well.
The 1860s were at the beginning of the 40 year long bicycle craze that lasted till 1900. Skiing was invented in 1862 additionally. Impressionism also became visible to the public eye.
In summary, there were many different advances in technology, biology, as well as several wars, in the decade preceding Jules Verne’s 20,000 Leagues Under the Sea.