The Saiyan race possibly has the greatest potential out of any species in the universe of Dragon Ball. Saiyans are a naturally powerful species that have multiple ways of gaining even more strength. They can transform into Giant Apes, achieve multiple Super Saiyan forms, and become stronger after surviving near-death experiences. Under the right circumstances and when pushed to the extremes, Saiyans have proven that they can be the strongest warriors in the universe. But what if the mightiest species was related to the race that was considered the weakest?
Saiyans and humans have a lot of similarities. This is strange considering that both species developed on different planets millions of miles away from one another. However, despite the distance between them, the two species are almost identical in terms of appearance and anatomy. The only real difference here is the monkey-like tail that Saiyans sport. There is a shocking amount of evidence to support the theory that Saiyans and humans are somehow related.
Saiyans and Humans are Genetically Compatible
Since coming to Earth, Goku and Vegeta have both started families and fathered children with humans. These children -- Gohan, Goten and Trunks -- were conceived and birthed without any issue. There were no mutations or complications, and all three children have grown up to be strong and healthy. This lends credence to the theory that Saiyans and humans share some DNA -- otherwise, they wouldn't be able to produce offspring. Neither Saiyans nor humans have been able to produce hybrid offspring with any other alien race besides each other. In canon, only humans and Saiyans can create hybrids with one another.
These hybrids, despite only having half Saiyan DNA, do not appear to be genetically weaker than their parents. After achieving Super Saiyan 2, Gohan was significantly stronger than his father for a period of time. When Future Trunks was introduced, he was much more powerful than his adult Saiyan father. Goten was even able to become a Super Saiyan at the very young age of seven. It seems like being half human and half Saiyan makes a person stronger than just being a full-blooded Saiyan. Maybe human DNA is the missing key to unlocking the full genetic potential of a Saiyan.
The hybrid offspring of Saiyans and humans are also fertile. Gohan was able to conceive a daughter, Pan, with his wife Videl. Even as an infant, Pan has displayed Saiyan levels of power. Characters have stated that she has the same great potential as Gohan. Even though Pan is more human than Saiyan, she still has all the power of a full-blooded Saiyan. All this proves that the hybrid species of humans and Saiyans is here to stay and grows stronger with each new generation. Saiyans and humans are remarkably compatible despite being so distant.
Saiyans and Humans Evolved From Primates
The connection between Saiyans and primates has always been strong in the Dragon Ball series. The tails that Saiyans are born with are the most obvious connections. Saiyans have the same level of control over their tails as monkeys do. They can use their tails to grasp objects and they aid with balance. The tails of Saiyans are also the key to their Great Ape form. Without their tails, Saiyans lose the ability to transform into their giant ape forms when they look at a full moon. This is the main reason why other species, like Frieza, call Saiyans monkeys or apes. It's highly likely that Saiyans evolved from some kind of primate.
This idea falls in line with current theories that humans also evolved from primates. This would mean that Saiyans and humans originated from the same ancestor species or at least a very similar one. It would also explain why Saiyans and humans look so similar when almost every other alien race is drastically different. If the two species shared the same evolutionary path, that only adds to the evidence that they are related in some way.
Saiyans and Humans Come From the Same Planet
The Saiyans typically have a great disdain for weakness, especially weakness in other Saiyans. They even have a tradition of sending newborns with low power levels to distant planets to conquer them. This is how Goku ended up on Earth as a child and had no knowledge that he was an alien. This tradition might explain how Saiyans and humans are related and how they ended up on different planets.
Saiyans and humans might at one time have been the same species or distant cousins at the very least. The weakest individuals of that species, the ones that didn't innately know how to use Ki or lacked tails, would have been reviled by the stronger individuals. These weaker individuals could have been exiled from their home planet and sent to Earth, where they became known as humans. It's possible that the original Saiyans simply forgot about their weaker relatives after a time or purposefully removed them from their history. This could have been the start of the tradition of shipping weaker members of the species to distant planets. This theory explains why humans and Saiyans look so similar and why they're able to produce viable offspring.