Science can explain the process, not how it came about.
Science can postulate how it came about, however. Even the old primordial soup hypothesis makes sense. A couple of billion years ago the earth had cooled sufficiently and was covered 70-75% with water. The water was not pure H 2 0 but contained lots and lots of naturally occuring chemical compounds. These compounds made trillions upon trillions of random connections with other compounds and formed more complex compounds. Some chemical compounds have the capacity to combine with other compounds that are subsequently bonded within the molecule, others cause chemical reactions without being consumed in the process, ie. like a catalyst. Here scientists are divided. Many hold that a self-replicating molecule akin to primitive RNA arose and this was the genesis of life. Others, correctly citing that this would be an exceedingly improbable (although not by definition impossible) event believe that independent mutually replicating molecules is where it all began and it was after that self-replicators appeared. Whatever the beginning was, there was obviously no life on the planet to eat the replicators and they were free to populate the oceans with myriad versions of themselves. Some replicators grew dominant because they broke down and bonded with other replicators. Most times the modification was fatal because it either reduced or eliminated the molecules' ability to replicate but a very small part of the time the new more complex molecules were improved in ways that gave them some kind of advantage in the replication game, and then natural selection was in play.