One way to look at it is that all life forms are similar on earth because all of them came from a single spark as theorized by Stanley Miller (
https://en.wikipedia.org/wiki/Miller–Urey_experiment). It is regarded as a groundbreaking experiment, and the classic experiment investigating the origin of life (abiogenesis). It was performed in 1952.
So far this is the only way for life to replicate on earth, found so far is the chemical bonding between Adenine and Guanine and Cytosine and Thiamine.
It is very much possible there are other complex molecules that can form a bond and when it breaks agains form a similar molecule but two of them. Maybe such a thing has happened on an alien planet. So if there is life on an alien planent, it is more likely they will be very different than life on earth.