The bats wing and our arm are "homologous structures" and are an example of divergent evolution. The bat and our common ancestor, most likely a small mammal, had the same number of arm bones as us and bats, and when bats/humans speciated (became new species), we kept the same number of bones. However, bats found it advantageous to grow their "fingers" very long in order to hold their wings, whereas we found it useful to grow our arms long, so we can reach and lift things.
All in all, the same bone number and similar structure shows that humans and bats shared a common ancestor, and our arms are "homologous structures".
