Original sin isn't necessary at all, only that many people have sinned.
No, the point of original sin there is that it means nobody can be blameless, even in principle, i.e. all people are tainted by Adam and Eve's actions. It leads directly to the idea that people need divine intervention to be absolved of that taint, which which justifies Jesus's sacrifice-for-redemption and underlies pretty much all of Christianity.
Why is evolution an issue in this? Why does it need reconciling with original sin?
All that is required for Jesus to redeem humanity is humanity as a collective has sinned. That doesn't mean that all are born sinful, just that all - or even just some - end up sinning. Somewhere in the NT is the line 'all have sinned and all fall short of the Glory of God'.
Why is evolution an issue in this? Why does it need reconciling with original sin?
Because Adam and Eve clearly never existed?
Yes. But why does it then follow that Christianity needs reconciling with evolution?
Are you implying that original sin (the idea that people are inevitably going to break their own moral codes) is incompatible with evolution? I don't understand why you should think this?
What?
When has original sin ever been an act? It's something you're born with, isn't it?