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When I am learning about evolution I barely see any source that explains how insects, mammals, reptiles and fish came to be. Can someone show me a source where I can learn about this? Especially insects and mammals - I want to know how those 2 distinct group came to be.

And I have a related question: can a fly evolve into a rabbit given enough time with the necessary condition? (I know the question sound stupid but please can someone explain me that)

I have a layman understanding of evolution. So please can any of your source or explanation be simple? Thank you.

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Re a fly evolving into a rabbit, theoretically yes, practically no. Insects (and invertebrates in general) made some evolutionary "choices" like having an exoskeleton & breathing through trachea that put serious limits on their maximum size. (And vertebrates, particularly warm-blooded mammals & birds, likewise have limits on their minimum size.) So to get a rabbit-sized fly (in Earth gravity & atmosphere) you'd have to make a bunch of major changes, with no obvious environmental niches for the intermediates.

But where the size limits and other constrants overlap, convergent evolution can produce remarkably similar results. I think perhaps the best example is the hummingbird https://en.wikipedia.org/wiki/Hummingbird and the hummingbird moths https://en.wikipedia.org/wiki/Hemaris They're about the same size, live in the same habitat with similar lifestyle, and from a distance are almost indistinguishable, yet evolutionarily speaking, they're about as different as animals can be.

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Insects and mammals diverged from one-another over 500 million years ago. For most of Earths 4.5 billion year history, life was single-celled, or organized into colonies of single-celled organisms. Likely sometime shortly before the Cambrian, multicellular animals evolved. The first multi-celled animals were likely a kind of colony that was not differentiated into tissues. It didn't have a head, or tail, or look much like an animal. From these simple undifferentiated, multi-celled building blocks, came most of our animal phyla, during a burst of innovation called the "Cambrian explosion". At this time, one lineage headed down a path that would lead to insects, and another lineage, toward mammals. Although theoretically, it is possible, given enough time (as in billions of years) for a fly to evolve into a rabbit, this is highly unlikely. Evolution works with what it starts with and modifies it under natural selection to favor fitness. So having six legs and wings works pretty well for flies, and having 4 legs and fur works pretty well for rabbits. The hurdles to go from one form to another, given that these lineages are already well adapted, would be enormous. It was much easier, 500+million years ago, to go from one colonial lump of cells to a rabbit, and another lump to a fly. As an example of how evolution works with what it starts with, virtually all tetrapods (amphibians, reptiles, and mammals) have 5 digits on their limbs. This is because they share a common ancestor that did. Not because 5 is somehow special. Another example, humans choke to death because we share a common ancestor with fish, and fish share a tube that connects both the mouth with their swim bladders and their gut. Swim bladders were modified into lungs, but the common connection from our fish ancestors remains (a really bad design, but we are stuck with it).

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The two groups that you mention belong to two separated branches in the animal kingdom. The fly and all insects are protostomes and the rabbit and all mammals are deuterostomes. In the first, the embryonic opening (the blastopore) becomes the mouth, while in the second, it becomes the anus. Wiki At first glance, it seems like a simple difference. But it became a total change in terms of body structure and allow the appearance of animals whit the nervous system in the back, like us (insect have a ventral nerve cord).

In theory, it is possible for a population of flyes to accumulate the mutations to become something similar to a population of rabbits. All the intermediate forms should be viable and able to reproduce them self.

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