The First human was a member of an exploration mission from the Earth Empire.
The humans made contact with the Sensorites, discovered that the Sense Sphere was rich in molybdenum and attempted to exploit its mineral wealth. Shortly afterwards the crew appeared to die when their spacecraft exploded. In fact they had faked their deaths and hid in the caves outside the Sensorites' city. They began adding atropine to the water supply in the hopes of killing them all and claiming the planet for themselves. Their scheme was discovered by the First Doctor and they were taken into custody by Maitland's crew. (TV: The Sensorites)