The term simian line origins in the fact that in primates (or simians) 'transversal' palmar creases resemble the usual condition.
The single palmar transverse crease was named as the simian line / simian crease because of the fact that the hands of 'simians' (a synonym for: 'monkeys' and 'apes') are usually characterised by the presence of various horizontal creases which transverse the full simian hand.
So, the word 'simian line' stems merely from its resemblance to the palmar creases which are supposed to be found in the hands of primates.
Quote from R.S. Bali:
"Paul Broca (1877) was the first to stir up interest among anthropologists in the simian crease. He introduced the ethnic element in creases. Cararra (1893) and Fere (1900) studied the frequency of simian creases in Italian and French soldiers, respectively. The single transverse palmar line, which runs across the palm, is typically for most simians and, therefore, gets the name "Simian Crease"."
By the way, researchers & experts in science often prefer to use other names to describe simian lines, merely because of the 'primate' connotation.
What have been the major milestones in the history of the simian line after R.L. Down discovered the significance of the 'single palmar transverse crease' in Down's syndrome in 1906? One of the key-issue to start with is the fact that the prevalence of the simian line significantly varies with enthnicity around the world ...more.
The hand of primates versus the human hand: