Why do rivers keep making dolphins? Four river dolphins from three ancient, separate branches of the toothed whale family tree share one body: a long, thin beak, tiny eyes, broad flippers and a quiet sonar.

It starts in the flooded Amazon forest with the boto, whose unfused neck and paddle-like flippers let it turn between drowned trunks, and whose quiet, rapid clicks are built for short range. In the Ganges and the Indus, a dolphin with lensless eyes sweeps the riverbed on its side, and the fossil Pebanista shows that its relatives once hunted in an early Amazon.

In the Yangtze, the 2006 survey found no baiji at all, the first whale or dolphin species likely driven to extinction by people. Off the Río de la Plata, the franciscana proves that fresh water is not what these dolphins share: it hunts in a murky estuary and along the coast.

Finally, the answer: light, sound, prey and space. Murky, cluttered water favors dolphins that hunt at close range, and over millions of years it pushed eyes, sonar, jaws and bodies in the same direction. We also look at what stays open, from the relict idea to the tucuxi that kept a short beak. Which murky river or coast do you think could shape the next dolphin like this? Tell us in the comments, and subscribe for more of the patterns evolution keeps repeating.

Chapters:
0:00 Four dolphins, one body
1:29 The boto of the flooded forest
2:54 A quiet sonar for short range
3:34 Isthminia and the boto's family tree
4:44 The side-swimming dolphin of the Ganges
5:17 Eyes without a lens
6:09 Quiet clicks in the Sundarbans
6:47 An ancient, separate branch
8:00 Pebanista, a giant of the early Amazon
9:10 The search for the baiji
10:40 The baiji's place on the family tree
11:17 How the baiji disappeared
12:35 The franciscana of the Río de la Plata
14:18 Are river dolphins relicts?
15:55 Why murky water shapes dolphins
16:25 Four pressures: light, sound, prey, space
17:58 Four dolphins, one answer
18:48 Inherited or shaped by the water?
19:13 The limits of the rule