The fossil record of rays/skates goes all the way to the Lower Jurassic (about 150 million years ago). Due to these animals having skeletons made out of cartilage, they leave little fossil record except teeth.
Skates belong to the family Rajidae, in the superorder Batoidea. There are more than 200 different species that are divided into 27 genera. The two subfamilies are Rajinae (hardnose skates) and Arhynchobatinae (softnose skates).
"So, how can you tell the difference between a ray and skate?"
First, rays are generally larger than skates. Skates usually have a prominent dorsal fin, which is small, if not absent, in rays. Rays are usually kite shaped or rounded, with whip-like tails that have spine(s) that have the ability to deliver a painful, and venomous, sting. Meanwhile, skates have a fleshy tail that lacks spines, therefore they protect themselves with large thorns on their backs and tails.
Skates also have small teeth, and rays have plate-like teeth (for crushing... prey). Their major difference lies internally though: their reproductive strategies. Rays are viviparous (bear live young) while skates are oviparous (lay leathery eggs, similar to shark egg capsules).
"So are they harmful to humans?"
Skates, which lack stinging spines, are completely harmless to humans! You might get a bit scratched up from the thorns due to improper handling, but nothing a band-aid can't fix in a quick minute.
Oviparous. Fertilized eggs laid in a protective case (mermaid's purse).
Fins have 2 lobes
Fleshy, lack spine
Have "thorny projections on their backs and tails for protection from predators" (seen on above picture, right)
Smaller than rays
Viviparous; bear young inside bodies and give live birth.
Missing or vestigial
Fins have 1 lobe (seen on above picture, left)
Whip-like with stinging spine(s)
Rely on their stinging spines or barbs for protection (seen on above picture, left)
"Plate-like teeth adapted for crushing prey"
Larger than skates