When did the left jab become the main weapon in the HW division?

When did boxers start relying on the left jab and using it as their main weapon? When I watch footage of James J. Corbett, Bob Fitzsimmons, James Jeffries, and Jack Johnson, I hardly ever see a consistent left jab. Their style looks like a completely different sport compared to modern boxing.

They seem to rush at each other — Jeffries even jumps in with body uppercuts — exchange one or two punches, clinch, get separated by the referee, reset at distance, and then rush in again. This pattern repeats over and over.

However, when I watch footage of Joe Gans, Terry McGovern, George Dixon, or other lighter fighters from the same era, they look much more like modern boxers. They throw jabs, crosses, and uppercuts, use recognizable combinations, and adopt stances that feel surprisingly modern.

I’ve also read boxing books written by Bob Fitzsimmons and James J. Corbett, where they explain the main punches — jab, cross, uppercut — very clearly. This suggests that they did understand these techniques in much the same way modern boxers do.

So why did heavyweight fighters of that era fight in such a different way compared to the lighter fighters of their time?