Did evolution happen in a linear way?

[rov1200]

If so how was it possible for species to survive when they are so dependent on other organisms or species to act in parallel.

ie. humans need gut bacteria
predatory animals need a foe
some viruses need bacteria

and literally applies to most living things!

[WaldoMcFroog]

"For human life to evolve you would naturally need a male and female. Take one away and life could no longer function and would quickly die out.

Therefore both genders must have evolved together as separate entities. This is a very simple argument but proves that evolution as we know it is faulty. "

This is idiotic. Males and females of *any* species are essentially genetically identical, which immediately puts the kibosh to that notion. Not only that you can point to things such as the fact that all male embryos are intially female. The reason they're one or the other is because of which genes are expressed; by switching on different genes, you could change the gender identity.

On that basis alone, one is forced to conclude that whether or not we understand the mechanism by which it initially arose, the genders must have necessarily evolved together.

Moreover, the reason humans reproduce sexually is because all mammals evolved from an ancestor which did so. And that ancestor itself evolved from something which reproduced sexually, etc etc. In so far as we are able to extract it, this is supported by the evidence from mitochondrial DNA. Sexual reproduction continues back down the line until you get to a single-celled eukaryote which can carry out mitosis (related to assexual cell division) *and* meiosis (a precursor to sexual reproduction); we therefore have the bridge between assexual and sexual reproduction. At this point and not before, there are various hypothesis as to why meiosis happens, but there is *no* question that it does.