Today, 37-year-old Parag Agrawal is Twitter's CEO — the youngest among the S&P 500, edging out Mark Zuckerberg. But just 10 years ago he was an engineer at Twitter.

Agrawal, who was announced as CEO after Jack Dorsey's surprise resignation on Monday, joined Twitter as a software engineer only a decade ago, in 2011. And it took him just six years to reach the C-suite as CTO. Agrawal made a name for himself by advancing the use of AI and machine-learning technology at Twitter, which helped boost the company's audience growth.