Back in 2014, Chuck Norris did a Q&A session on YouTube. He made an interesting comment about himself and Bruce Lee.
Norris is well into his senior years at this point, but he was a fearsome competitor in his prime. He holds multiple black belts in Judo, Taekwondo, and even Karate, among others. In fact, Norris even dabbled in Jiu Jitsu decades ago.
Bruce Lee similarly knew his way around various martial arts. It’s not a stretch to think that both could have been champions in MMA if it had existed in their primes.