Secret Invasion: 10 Things You Need To Know About The MCU's Next Big Event

Even though the Skrulls' most identifiable trait is their shape-shifting abilities, that wasn't always the case. Over a billion years ago, space gods called the Celestials performed scientific experiments on the Skrulls because... well, what else is a space god going to do? Anyway, these procedures endowed the Skrulls with metamorphosing cells.

Unfortunately, there were side-effects. Not only are Skrulls susceptible to cancer, leukaemia and tuberculosis, the experiments rendered them evolutionary stagnant. Worse still, the Skrulls have great difficulty reproducing. And when their home world was destroyed, their extinction seemed like a very real possibility. From their point of view, there was only one way to survive: Earth.

The Skrulls can reproduce by mating with humans or by using human DNA to create viable clones.

Taking over the world isn't about revenge - okay, it totally is - but the Skrulls are also taking over the world because they NEED humans to survive. It's easy to paint the aliens as generic baddies, but they don't see their actions as wicked. Although their invasion will decimate our world beyond repair, it will preserve their species, which they see as the highest priority imaginable.