Final answer:
Indentured servants, who were European immigrants working under contracts, were the main labor source in the English colonies up until the mid-seventeenth century before the shift towards slavery.
Step-by-step explanation:
The group of people that provided the primary source of labor in the English colonies up until the mid-seventeenth century were indentured servants. These were typically European immigrants who agreed to work for a set period of time, usually four to seven years, in exchange for passage to the colonies and the hope for a new life thereafter. A significant number of people, estimated to be up to 100,000, mostly young men in their twenties, came to the English colonies under this system. Despite the harsh conditions and high mortality rates, those who completed their service could become respected members of colonial society, and some even achieved wealth as planters. The extensive use of indentured servants preceded the shift toward greater reliance on African slaves, which intensified in the late seventeenth century as the demand for labor increased, and the availability of indentured servants declined.