In the past, gender has been described as binary, as in you sit on one of the two ends of the scale: male or female. Over time, gender has evolved and is now perceived as a social construct. The best method of describing gender is imagining it as a spectrum; no person, even cisgender people, recognises as wholly feminine or wholly masculine, so the binary view is not fully accurate. The spectrum identifies that gender is a fluid concept and that even people who identify as the same gender do not have matching expressions. However, the emergence of non-binary genders has produced arguments of whether these genders exist...which they always have! Past cultures have indicated the recognition of 'a third gender' - such as two-spirited people in indigenous tribes - indicating that the idea of a spectrum has always existed, but never been accepted by mainstream culture. So, I though I would use this platform to explain this history of gender:

Male and female have always been the social norm for gender throughout history. Gender was originally defined by the chromosomes you were born with, so many people believed that gender could be only XX or XY (female or male). This was before the discovery of XXY genotypes and such. 

But the existence of non-binary people has been noted and catalogued through history. 

The earliest recording of non-binary people is in Mesopotamia; their mythology makes multiple references to non-binary people (such as priests and those who performed religious duties). 

Many Hawaiian cultures are third gender persons that traditionally play a spiritual role within the culture. 

The Buddhist Tipikaka documents note 4 genders: male, female, ubhatoyanjanaka (people with both male and female characteristics) and pandaka (no current English translation). 

Many Native American populations recognise a third gender. European anthropologists refer to these people as 'berdaches', which Native Americas consider a slur, so the term 'two-spirited' was adopted. 

Across the Indian subcontinent, genders outside the binary are collectively known as Hijra (not completely male or female). 

Any Questions? Don't hesitate to ask!