A bridge of land
What Is an Isthmus?
A strip of land a few kilometres wide can decide which animals exist on a continent, and where the world's ships go.
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Step 1 of 5
It joins land and separates water.
An isthmus is the exact opposite of a strait. One is a narrow gap of water between two seas; the other is a narrow gap of land between two coasts.
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Step 2 of 5
Animals cross as soon as it appears.
When an isthmus rises, everything on both sides starts walking. Whole groups of animals move into land they had never reached, and many of the residents lose out.
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Step 3 of 5
Sea life is cut in two on the same day.
What is a road for land animals is a wall for sea ones. Populations that used to mix are separated, and over time each side drifts into a different kind.
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Step 4 of 5
Ocean currents get rerouted.
Closing a gap between two oceans forces water that used to flow through to turn instead. That rerouting can change the climate of continents far away.
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Step 5 of 5
People cut through them.
A narrow neck between two oceans is worth an enormous amount to shipping. Where the land is narrow enough, canals have been dug straight through.
When one famous isthmus rose about three million years ago, it triggered one of the biggest exchanges of land animals in the fossil record.
The short version
An isthmus is a narrow strip of land joining two larger ones: a bridge for animals that walk and a wall for everything that swims.
Try it yourself
Find a world map and look for the narrowest places joining two continents. Both have canals cut through them.