How was Okonkwo able to achieve such a high rank within the tribe when his father died a penniless man? How is this different from colonial-era European culture?

Okonkwo such a high rank because of his own personal achievments. He was a great wrestler, became a wealthy farmer, and had two titles. In European culture a man inherited his position, wealth, and title. In Okonkwo's culture, what your father did or had has no bearing on how you are seen.