Simplify the following text for a Grade 12 native English-speaking person.
Original: Since little is known of them , they remain controversial among scientists since the molecular clock in humans has determined that humans and chimpanzees had an evolutionary split at least a million years later .
Simplified: Since little is known of them , they remain controversial because the molecular clock in humans has determined that humans and chimpanzees had an evolutionary split at least a million years later .