Humans are thought to have evolved from other lifeforms in a process known as "evolution". Some scientists say that humans can't evolve much further because of certain factors, although it must be noted that some disagree.
Initially, the first forms of life were really small, like a few cells or something, but then they started getting bigger and turned into stuff like plants and insects and fish, which then became more and more complicated and even bigger.
For instance, about 100,000,000 years back, creatures were really big and called "dinosaurs". They were everywhere and there weren't any humans at all, but then all the dinosaurs disappeared for some reason. Some say it got too cold for them, some say it got too hot; whatever, they all died. Of the species that were left behind after whatever it was that happened, some sort of amphibious creature probably adapted itself to living exclusively on land, that became something like a monkey and then evolved into a human or something.
Then, a few hundred thousand years ago or so (or maybe about a million), humans learnt to do a few new things, like effectively communicate with one another and make stuff. At some point during this period they also worked out how to make fire, which allowed them to make things hotter. With the stuff they made they could team up against some of the the other species and kill them, despite not being as big or strong. Then someone had the idea to put some of the things they'd killed on the fire (in a process later called "cooking"), which made things easier to eat and allowed humans to get the stuff they needed to carry on living more effectively than the other species and anyway, humans became dominant.
Humans are responsible for the main body of all the stuff that can be seen on the Earth, but - even working together as a species - have thus far had little overall effect on its main mass and composition and stuff. However, despite their dominance of the planet for what is a comparitively short period of time on the cosmological scale, some say that humans have managed to ultimately render the Earth uninhabitable by digging up stuff and setting it on fire, so beginning the process known as global warming, although others disagree. Who knows? The future of humanity is a subject much discussed within circles of experts and cranks alike.