Gene Duplication and the Dawn of Vertebrates

Vertebrates acquired more than just spinal cords running down their back during the Cambrian explosion. With some tinkering to their genetic tool kit, they evolved eyes, complex brains, and skeletons. In the process, vertebrates became powerful swimmers and excellent hunters and have remained the dominant predators of the ocean and land ever since.

The oldest known vertebrate fossils–lamprey‑like creatures found in China–date back to the midst of the Cambrian explosion, 530 million years ago. In order to understand how those first vertebrates emerged from their ancestors, biologists have studied our closest living invertebrate relative. Known as the lancelet, it’s not a very impressive cousin. It actually looks like a headless sardine pulled from a can. The lancelet starts out life as a tiny larva, floating in shallow coastal waters and swallowing bits of food that drift past it. When it grows to be half an inch long, the adult lancelet burrows into the sand, sticks its head up into the water, and continues to filter‑feed.

But as unassuming as the lancelet may look, it shares some key traits with vertebrates. It has slits near the front of its body that correspond to the gills of fish. It has a nerve cord running along its back, which is stiffened’ by a rod called a notochord. Vertebrates have notochords as well, but only while they are embryos. Over time, the notochord withers away as the spinal column grows larger.

In other words, certain pieces of the vertebrate body plan had already evolved in the common ancestor of lancelets and vertebrates. Yet lancelets also lack much of the anatomy that make vertebrates so distinct. They have no eyes, for example, and their nerve cord ends in a tiny bump, not in a proper mass of neurons one would, at first glance, call a brain.

But it’s possible to see precursors of brains and eyes in lancelets. The lancelet can detect light with a pit lined with photosensitive cells, and these cells are wired up like a retina in a vertebrate eye and connected to the front of the nerve cord in much the same way as our eyes are to our brains. That tiny bump at the front of the lancelet’s nerve cord may consist of only a few hundred neurons (our brains have 100 billion), but it is divided into simplified versions of parts of our own vertebrate brains.

The similarity between the lancelet’s nerve cord and vertebrate brains extends to the genes that build them. Hox genes and other master‑control genes that map out the vertebrate brain and spinal cord do the same job in the lancelet embryo, in almost precisely the same head‑to‑tail order. In the cells of the developing lancelet eyespot, the genes are the same as those that build a vertebrate eye. It’s a safe bet that the common ancestor of lancelets and vertebrates had the same genes for constructing the same basic brain.

Once the ancestors of vertebrates and lancelets branched apart, our ancestors went through an extraordinary evolutionary experience. Lancelets have 13 Hox genes, but vertebrates have four sets of those genes, each arranged in the same head‑to‑tail order. Mutations must have caused the original set of Hox genes to be duplicated. After they evolved into four sets, the new genes met various fates. Some of them went on carrying out the jobs of the original Hox gene. But other copies of Hox genes evolved until they were able to help shape the vertebrate embryo in new ways.

Thanks to this explosion of gene duplication, our ancestors began to evolve more complex body plans. Vertebrates were able to grow noses, eyes, skeletons, and powerful swallowing muscles. At some point early in vertebrate evolution, the Hox genes that controlled their head‑to‑tail development were borrowed to help build fins. Fins helped vertebrates control their swimming and maneuver more effectively than their lancelet‑shaped ancestors.

Instead of passively filtering food out of seawater, early vertebrates could now start hunting. Able to chase down big animals, they could evolve to be bigger themselves. Thanks to their genetic revolution, the early vertebrates eventually gave rise to sharks, anacondas, humans, and whales. Without those new Cambrian genes, we might still be like the lancelets, our tiny brainless heads still swaying in the tides.