Cabbage, broccoli, cauliflower, kale, brussels sprouts, collard greens, kohlrabi, and several closely related vegetables originated from the same plant species wild mustard.
About 2,500 years ago, wild mustard only grew in some parts of Europe and the Mediterranean. Its taste varied greatly, depending on where it grew. Ancient Romans and Greeks soon realized that they could plant it for food. They engaged in selective breeding by planting seeds from wild mustard with larger leaves. The result was the vegetables that we call kale and collard greens.
Selective breeding continued in the 1600s when people bred wild mustard with bigger leaf buds. The result was a new vegetable covered with lots of leaves. This was the first cabbage. Wild mustard selected for its bigger stems became kohlrabi, the ones with small heads became brussels sprouts, and the ones with big flowers became broccoli and cauliflower.
The hybridization of wild mustard and its derivatives continued up to the 20th century. In 1928, Russian biologist Georgii Dmintrievich Karpechenko crossed a radish with cabbage to produce what he called the rabbage. The rabbage should have been impossible because the radish is not related to the cabbage.
However, the plant never caught on because it failed at being either a radish or a cabbage. In 1993, a Japanese company crossbred broccoli with kai-lan to create broccolini. Kai-lan is not popular in the US. It is a derivative of wild mustard and is the Chinese version of broccoli.