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Do we start life as females?

The origin of the idea

The notion that human life begins as female has been around for centuries. As early as the 17th century, researchers looking at embryonic development noted that the early gonads of mammalian embryos appeared female or gender neutral. It wasn’t until weeks later that the gonads differentiated into male or female anatomy. This observation led to the idea that perhaps all human embryos start off as essentially female, with masculinization occurring later.

This concept gained more traction in the 20th century as scientists explored hormones and genetics. Research on conditions like congenital adrenal hyperplasia provided clues that exposure to androgens (male sex hormones) is essential for the development of male genitalia. Without this hormonal influence, the thinking went, perhaps the default pathway is female.

Fetal development of reproductive organs

To evaluate this idea, it’s important to take a close look at how reproductive organs develop. In the first two months after conception, the fetal gonads are identical and undifferentiated in males and females. They contain two key cell types:

– Supporting cell precursors – which can become either Sertoli cells (in males) or follicle cells (in females)
– Germ cells – which become sperm in males and eggs in females

At around 2 months gestation, the differentiation process begins. A gene on the Y chromosome called SRY kicks off a cascade of events that lead the undifferentiated gonads to become testes. The SRY gene triggers supporting cells to become Sertoli cells, which nourish and support the development of sperm. Without the SRY gene (in female fetuses), the gonads instead develop into ovaries.

So while the early gonads are identical, they already contain the cellular precursors needed to develop into either testes or ovaries. The SRY gene simply flips the switch to send them down the male pathway.

Hormonal influences on genital development

The differentiation of the internal gonads into male (testes) or female (ovaries) then influences the development of the external genitalia. This happens through the secretion of key hormones:

– In males, the testes secrete testosterone which triggers the development of male genitalia (penis, scrotum)
– In females, the absence of testosterone allows the genitalia to develop into female anatomy (clitoris, labia)

So do female genitals represent some kind of “default” pathway? Not exactly. The latest research suggests the undifferentiated early genitalia may actually be poised right in the middle between male and female. Hormones from the testes or ovaries then actively push development in one direction or the other.

In other words, rather than a female default, there may simply be a period early in development where genitalia are plastic and responsive to hormonal cues that determine their final anatomy.

Sex determination across mammals

If female development was truly the default among mammals, we might expect the genetic triggers that initiate male development to be different across species.

In fact, the SRY gene is unique to humans and closely related primates. Other mammals use different genes to initiate testes development. For example:

Species Male sex determining gene
Mouse Sry
Horse SXX1
Monotreme mammals AMH

The fact that diverse male triggers exist argues against the idea of a universal female default among mammals.

Sex determination in non-mammals

Looking beyond mammals reveals even more variation in genetic sex determination. For example:

– Birds have a ZW sex determination system. Males are the homogametic sex (ZZ), while females are heterogametic (ZW). The W chromosome contains a gene called W-linked DMRT1 that triggers female development.

– Many reptiles lack sex chromosomes altogether. Sex is determined by environmental factors like incubation temperature.

– Fish exhibit an amazing variety of sex determination systems. Some fish species even switch sex in response to environmental cues.

Across the animal kingdom, genetic sex determination clearly takes many forms. There is no universal default to female development.

Is male development just a modification of female?

Given the diversity of sex determination systems in nature, it’s reasonable to ask whether male development is just a modification of the default female pathway. Is the SRY gene, for example, simply overriding an otherwise female trajectory?

Increasingly, the evidence argues against this view. As mentioned, the early undifferentiated genitalia seem poised between male and female rather than intrinsically female. And on a genetic level, the SRY gene doesn’t merely inhibit female development, it triggers a completley separate cascade of gene expression and hormone signaling that drives male gonad formation.

Rather than think of male as a diversion from a female default, it may be more accurate to say both pathways develop from an initial undifferentiated state. Sex determination drives development in one direction or the other from this starting point.

Implications for transgender identity

The idea that female development represents some kind of default state has interesting social implications. Specifically, it has been used to argue that transgender identity has a biological basis in early developmental pathways.

The thinking goes like this: If male development is just a modification of an underlying female trajectory, perhaps a “male” brain could develop in the context of a female body (or vice versa) under certain circumstances. This reversal would then lead to transgender identity.

However, as discussed above, the latest science indicates sex determination is far more complex. There is no universal female default. Both male and female pathways represent substantial, intersecting developmental programs, not a diversion from one intrinsic starting point.

Of course, none of this rules out biological factors contributing to transgender identity. But the notion of femaleness as a default clearly oversimplifies the science of sex determination.


While the idea that “female is the default” contains elements of truth, human development is far more complex. Modern biology reveals many possibilities across evolution for how genetic programs and environmental cues direct sex determination and gender.

Rather than a female default, the earliest stages of development likely represent a state of potential that can progress down male or female pathways given the proper signals. Defining one path as default oversimplifies this intricate biological process.