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As far as we know, life has been on this planet for about 4 billion years. Of that time, about 3 billion years was nothing but single-celled organisms reproducing by binary fission. The multicellular and sexually reproducing organisms have only been here about 1 billion years. We are the newcomers. 

What that tells us is the binary fission is indeed a very successful strategy to survival. Why? Because what it lacks in diversity it makes up in sheer *numbers*. Bacteria (and other single-celled organisms like cyanobacteria) are all basically clones of each other with small changes, but binary fission is such an easy way to reproduce (none of that finding a mate, buying them dinner, or building a nest for them stuff), that they reproduce by the *billions*. So even though there are very little chances for variation from one bacterium to its 'daughter' cells ... they reproduce so fast that the chances of some variation appearing that lets them adapt to the next toxin, antibiotic, or inocculation, are pretty good. 

So single-celled organisms evolve (change over time) ... but they are not very innovative. That is why in over 3 billion years while they were the only form of life on the planet, there was *incredible* diversity as far as the different *types* of bacteria that formed, but after the early invention of photosynthesis, there was little in the way of major innovations ... other than the development of the nucleus and the basic structure of the eukaryotic cell (probably through the discovery of symbiosis, where various cells found advantage to co-existing as organelles in a single cell). 

But then about 1 billion years ago two major developments happened, and it is probably no coincidence that they happened at about the same time (one of them leading immediately to the other). The first was sexual reproduction, and the second was multicellularity (cells living together in colonies of ever more specialized cell types). 

Sexual reproduction is a lot slower (because it requires two gametes to fuse to produce a new zygote) ... but the result is far faster innovation *per generation*. That's because with sexual reproduction, every new individual is different from *either* of its parents. So every new individual is a new "experiment" offered to natural selection. A new opportunity for some significant change that, if advantageous, spreads into the population and becomes the new norm. And so the chances for greater and greater complexity are much more likely. And that is why there are so many sexually reproducing species ... because if there's one thing they're good at, it's making more of them. 

So in summary ... binary fission produces *HUGE* numbers of individuals and generations, but very slow evolution *per generation*, and not much escalation in complexity. But sexual reproduction produces in smaller numbers, but with very high innovation, and thus faster evolution (more change per generation), and thus new opportunities for whole new levels of complexity.