No they are not human, although they look identical. They are a species which are known as Sebaceans, and are both Peacekeepers,the largest Sebacean polity in the universe. Throughout the series the words "Peacekeeper" and "Sebacean" are often used indiscriminately but the main characters do meet a number of Sebaceans who are not Peacekeepers. While Sebaceans and Humans are visually indistinguishable from each other Sebaceans, particularly Peacekeeper Sebaceans, do have some notable differences.The first major difference between humans and Sebacians is revealed in Season 1 Episode 2 "Exodus from Genesis", namely that Sebacians are cold blooded. Another of these differences, which plays a crucial role in the series, is that Peacekeeper women have some limited control over their reproductive abilities. Because of their combat duties Peacekeeper women have been genetically engineered so that they can prevent a fertilized egg from beginning to gestate. It is generally implied throughout the series that Sebaceans are physically superior to humans being tougher, longer lived, and stronger. The last is portrayed rather inconsistently. Often Aeryn Sun is portrayed as at least as strong as Crichton due to her status as a Peacekeeper. However Crichton seems to have no trouble fighting male Peacekeepers, who would presumably be at least as strong as their women.In the Peacekeeper Wars miniseries, it is revealed that thousands of years ago, the original Sebaceans were brought to what is now Peacekeeper Space from a distant and unexplored part of the universe. The idea was to introduce a race no one had ever encountered before to become Peacekeepers--a race with no pre-existing loyalties to bring their neutrality into question. The overwhelming implication is that these original Sebaceans were brought from Earth.
They don't. One of the alien technologies is "translator microbes." These are microscopic organisms which live in a sentients brain and automatically translate (almost) any language he hears into his own native language. We hear the alien characters speaking in English because Crichton was injected with translator microbes and he hears them speaking English.As an interesting addition, in the series when Crichton hears other human languages they are translated as well.If Crichton meets another species who have no translator microbes it is unclear how they allow the other species to understand what Crichton says or how Crichton understands them when there are no translators in the other species brain decoding what they are thinking when they speak and transmitting that information to Crichton's translators.