The story
Kelp are large brown seaweeds that grow in cool seas and anchor to the rocks with root-like holdfasts. Their fossil record is thin because soft algae rarely fossilize, but fossil holdfasts found in the Pacific Northwest of North America date to about 32 million years ago, in the early Oligocene. Genetic studies of kelp suggest the group diversified later, in the Miocene, as the northern Pacific cooled.
Kelp need cold water rich in nutrients, and the global cooling after Antarctic ice formed created such waters along many coasts. Giant kelp can grow about half a metre a day and reach tens of metres, and the forests support fish, urchins, sea otters and many invertebrates. Sea otters, which control sea urchins that graze kelp, evolved about 5 million years ago, and their removal in historic times caused urchin outbreaks. Kelp forests may also have helped early humans move along the Pacific coast, an idea called the kelp highway hypothesis.
Why it mattered
- Kelp forests became one of the most productive habitats in cold seas, supporting rich food webs.
- They gave rise to marine animals such as sea otters and the huge Steller's sea cow, which fed on kelp.
- The kelp highway hypothesis ties coastal kelp to the early peopling of the Americas.
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