In The Power, how did the skein originate?

In Naomi Alderman's speculative sci-fi novel, adolescent girls and women across the world develop the ability to conduct an electrostatic force through their hands. This power is generated through a striated muscle near the collarbone referred to as a 'skein.' Skeins are predominately found in biological females, although there are rare cases of males possessing skeins. For males, this is considered a genetic abnormality.
The development of skeins has been traced to a drug manufactured by the US and the UK during World War Two. A chemical referred to as Guardian Angel entered global water supplies, which was designed as an antidote to nerve gas. The novel does not specify how the drug changes neuromuscular function, only that it is largely a sex-limited ability.