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One of the very basic facts that Darwin pointed out was about the life span of organisms. Organisms with smaller lifespan reproduce quickly and hence variations are produced faster. This helps in faster evolution.

One of the main goals of living organisms is to reproduce, this fact being the backbone of neo-Darwinism. For organisms with longer life span, say humans, the amount of time required for new generations to come up, as well as the energy requirement is high, much higher if compared to bacteria. Then why did evolution of complexity arise if bacterial life forms could survive in harsh conditions and reproduce and mutate faster?

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    $\begingroup$ Successful lineages survive. That doesn't mean they are restricted to minimally long generation times. All that is required is that they compete sufficiently in the struggle for life. Sure, they may adapt slower, show lesser variation, require more energy for sustenance, etc. But it works as an evolutionary strategy, and large animals not being extinct are a testament to this. I don't see the difficulty of the question, sorry! Additionally, there are advantages that come with size. $\endgroup$ – S Pr Jul 17 at 9:34
  • $\begingroup$ This is a good, but very broad (and complex^^) question. Bacteria and humans are worlds apart (and both complex in their own way). If you mainly meant complexity of a single organism, the link S Pr gave should give a good overview, this wiki article mentions a couple of mechansims that can affect complexity. I'd recommend you to focus on complexity in a specific trait (like size), since otherwise a proper answer might not even fit in a textbook. $\endgroup$ – Nicolai Jul 17 at 9:44
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The simple answer is that the evolution of large, slowly reproducing organisms is not preferred: it is simply not selected against.

The key mistake in your thinking is this statement:

One of the main goals of living organisms is to reproduce

Most living organisms have no such goal, they simply take actions that have, historically, led to the continuation of their lineage.

In a complex world, there are many different strategies that can lead to survival and propagation of an organism, and cooperation between cells is one of them. Even simple bacteria often form large multicellular aggregates, which can provide physical advantages for their members over dissociated individuals, such as resistance to physical damage and formation of protected microclimates. The inside of your body is another such protected microclimate. So multicellularity can be quite advantageous.

As for limiting reproduction: remember that the closest competitors every new organism has are its own relatives, who occupy the same space and compete for the same resources. Even bacteria will often limit their reproduction when resources are limited. Reproduction also takes significant resources. A faster reproduction then, is also not necessarily advantageous for long-term survival.

Bottom line: neither large nor small, fast nor slow is preferred. Instead, the study of evolution predicts that we will see what we do see: a multiplicity of forms and strategies adapted for different niches in the ecosystem formed by their interactions with other species and the external environment.

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  • $\begingroup$ Also, we could easily argue that the large & slowly reproducing life forms are not PREFERRED, they just manage to survive. There are many more rabbits & mice than elephants & whales, they're outnumbered by insects, which are vastly outnumbered by microorganisms. $\endgroup$ – jamesqf Jul 17 at 18:48

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