If we want to , we can trace the chain of human descent back to the appearance of vertebrates, or even to the photosynthetic cells that lie at the start of life itself.

• 如果我们非要这样做的话，我们可以把人类的沿袭之链追溯到脊椎动物的出现，或比它更早的光合细胞，这些细胞正是生命的起源。