As opposed to an invisible force which created the planet and all its creatures in 7 days? You can not know what happened at the start of the universe and there is still so much science has left to find out. Yes, earth does have an ecosystem but how do you know it's "extremely efficient", when there could be better planets out there? You know, in the millions of stars that are left to explore.
Millions...most estimates put the number of stars in the Milky Way alone in the hundreds of billions.
Most estimates also put the number of galaxies in the universe in the hundreds of billions.
Of course, there is no way to know for sure, since we don't really know dick about the universe and we are absolutely insignificant in it. Most of us (as evidenced by this thread) still think we are pretty smart, though.