Some Assembly Required summary

Scientists have long believed that embryos, and the process of transition from egg to adult, hold the secret to understanding the differences between species. But the first real breakthrough came in 1865, when French scientist Auguste Duméril peered into his aquarium and was met with a shocking sight. 

Several months earlier, Duméril had introduced a species of salamander from Mexico. What confronted him now were two types of salamanders – the parents had given birth to what appeared to be an entirely different species.

If learning the truth about your sea-squirt ancestors upset you, you might want to skip this next part.

You probably know that DNA is passed from parent to child. What you might not know is that, over the four-billion-year history of life, it has also passed from ancestral species to descendent species. That means you and a sea squirt share some of the same DNA.

For Darwin, natural selection was the motor for evolution, and individual variation was therefore the fuel. But, of course, individual variation can also mean mutation.

Hearing the word “mutant” might trigger images of sci-fi Frankenstein monsters. But for Darwin it was a little less dramatic. A mutant is just an individual that has a trait that looks or behaves differently from the norm. If that trait makes the individual successful, they will pass it on to the next generation. If the trait harms their chances of success, it will likely die out over time. 

Given all that humans have been able to accomplish, you’d be forgiven for thinking that our bodies are far more advanced than those of our closest relatives – and certainly more so than fish or plants. But if the sea squirt teaches us anything, it’s that our arrogant pride at being the world’s top predator is unfounded. 

Nowhere is this more obvious than in our genome. It seems reasonable to assume that our genome is a well-oiled machine where the chromosomes are in perfect order and everything works together as elegantly as a world-class orchestra. But that assumption is wrong.

As we’ve learned, the sea squirt is the common ancestor for all animal life on Earth. But billions of years before the sea squirt, another revolutionary event laid the foundation for life as we know it. 

The earliest living organisms on Earth were single-celled creatures. Over time, some of these organisms started to produce oxygen as a waste product, and the Earth’s atmosphere began gradually filling with oxygen. After billions of years, a different microbe that took its energy from oxygen began to flourish. When these microbes teamed up with a bacteria that could produce complex proteins, the formation of bodies became possible.