Human capital is the attempt to identify the investment within the human, as opposed to the more socially centred notions of capital perpetuated by Communism and other such economic systems. It was developed by Friedman in the 1960's. However, it has not necessarily produced the legacy that was envisaged:
Friedman envisaged a society in which we’d all be wealthy, thriving entrepreneurs. What we got in reality was a pay cut, reduced holiday or sick leave, a chronic skills deficit, credit-card debt and endless hours of pointless work. If anything, the story of human capital theory in Western economies has been about divesting in people, not the opposite.
That’s because it was born within an extreme period in 20th-century history, when many believed that the fate of humanity was hanging in the balance. It should therefore be approached as such, a rather eccentric and largely unrealistic relic of the Cold War. Only in that highly unusual milieu could mavericks such as Hayek and Friedman ever be taken seriously and listened to.(Source)