People once believed the sun circled the Earth. It was a perfectly sensible model by which they understood the universe. The important word here is “model”. At the time, almost everyone thought that this was accurate. As with our contemporary idea of gender, the model closely matched the observation of physical “reality”.

The philosophers and theologians of the middle ages believed that the sun went around the Earth and that to suggest otherwise was heretical, ungodly and just really, really stupid. Imagine scholars of that age writing on the internet about new theories that the Earth went around the sun – these opinions would read a lot like most of the comments we get today about gender. Basically: “This is just ignoring common sense”; “What is the world coming to?” and “Let’s burn these queer freaks.”

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Which they did.

The middle-ages model was based on the principles of Ptolemy, and hundreds of years of celestial observation. Everything worked perfectly well until Copernicus and Galileo gave us the new and exciting heliocentric model. Copernican thought was revolutionary, but then we had Newton, and then Einstein, Bohr and Hawking. We keep observing the universe, and we keep changing the model to fit the observation, not ignoring the data that doesn’t fit. We could also still use geocentrism if we wanted – we would be able to predict solar and lunar eclipses, roughly, but nothing would work properly.

The model of male and female is based on thousands of years of observation of genitalia and the binary qualities of sexual attraction and reproduction. Those were the externally observable facts. They correlated. And, yes, we could remain happy with that.

But now we are gathering observations from people who feel profoundly uncomfortable presenting as the gender they are classified in. We know that somewhere between 0.3% and 0.5% of the world’s population experience gender dysphoria and don’t feel they “fit” with a binary model. Are millions of such people wrong? Or is the current model wrong?

I identify as trans and bisexual – except that bisexual implies you like both sexes. I’ve recently realised I can’t really call myself that: I’m running out of sexual identities because they all rely on this idea that humans are either one kind or the other – gay or straight. I am attracted to all kinds of human, beyond narrow binaries of sexuality and gender.

A “single spectrum of human” is an idea that we already apply to race, neurology, and other aspects of life: we are all human, all humans are equal, all humans are different.

Many indigenous and non-western cultures have long acknowledged this. Trans identity is not new – it is only a surprise because we suppressed it for so long. People learn to hide it from early childhood, and from themselves.

We have a cultural precedent here as well, with left-handedness. Being a “southpaw” was considered pathological and untrustworthy all the way into the 20th century. Yet when it became acceptable, many more people became lefties, because they were hiding their left-handedness to begin with. This was most immediately visible in the classroom, where a new generation no longer had to hide their aberrant ability. We even had ambidextrous people. When stigma was reduced, kids were the first to discover it was OK to be left-handed – or both. This correlates to what we see today with trans. Amazingly there are ways in which left-handers are still stigmatised and discriminated against in areas such as design, often in unconscious ways. It is very hard to rid ourselves of an incumbent model and structural bias.

Changing our idea of gender is more challenging because we resemble the men who opposed the Copernican revolution: our society is heavily invested in the binary model that we believe to be true. It is much easier to disenfranchise people who break from the norm, or to say that millions of people are mentally ill (whatever that means), than to accept the existence of transgender people as evidence that our current model is flawed.

The ancient model that divides us into two distinct “sexes” is deeply ingrained. As a trans person, I prefer an “all human” model: we all identify individually. We should accept this. If we have to adjust the words we use or build our toilets differently, then we should. I cannot believe that kids should be suicidal because I, as a taxpayer, don’t want schools to change some of their facilities. We know that isn’t the true reason for resistance. It’s because we don’t want to believe the model is broken. We don’t like change. These are structural things. It is this fear of change that manifests in our newspapers each week as a desire for trans people not to exist at all.

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You may still think, “Well, that’s fine, clearly you do exist – but men are men and women are women.” Which is OK. You are exercising the right to your own beliefs, and to treat people differently based on those beliefs. That’s called a prejudice and acting upon it is called discrimination. For transgender people, prejudice is often tolerated if unspoken, and yet that is changing. The progress of civilisation has mirrored the slow erosion of “commonsense” prejudices. The sun doesn’t go around the Earth, the colour of your skin does not indicate your intelligence, “women” should not be paid less than “men” for the same work.

People occupy a broad circular spectrum of behaviours that are boyish, girlish, or something else altogether. Everyone is an individual human. How each of us define ourselves should be our own choice. Spreading that idea is up to everyone.

• Tea Uglow is creative director of Google’s Creative Lab in Sydney. Tea is speaking at WOW: Women of the World festival at London’s Southbank Centre, which runs from 7 to 11 March, supported by Bloomberg