Yet another piece of the puzzle could be unusual proteins called syncytins These proteins help cell membranes fuse together.

另一个难题可能是称为合胞素的不寻常蛋白质。这些蛋白质帮助细胞膜融合在一起。

What's interesting about syncytins is that they didn't evolve in mammals; they've been part of our genomes since before therians were even a thing.

合胞素的有趣之处在于它们不是在哺乳动物中进化而来的；在兽人出现之前， 它们就已经是我们基因组的一部分了。