What share of all humanity is alive right now?

No, and it is not close. Approximately 109 billion people have been born and died, against roughly 8.0 billion alive in 2022, so the living account for about 6.8% of all humans ever born. The myth survives because exponential population growth charts look dramatic, and a chart of population at a point in time is a different animal from a running total of births.

The Population Reference Bureau method starts from a notional pair of humans in 190,000 BCE. Researchers set population levels at waypoints such as 2 million people in 50,000 BCE and 300 million in 1 CE, using archaeological evidence, historical records and demographic models, then apply estimated birth rates to the gaps between them and sum the lot, which is where the ~117 billion comes from. Uncertainty in the prehistoric stretches is large and openly acknowledged, while the modern-era estimates rest on census data and are firm.

Around 1800, on PRB's figures. The running total stood at about 97.6 billion in 1750 and 101.6 billion in 1850, so the 100 billionth birth falls early in that century. Since 1900 another 12.5 billion or so have been born, about 11% of everyone who has ever lived, which is the demographic transition showing up in the arithmetic.

Anyone born in the last century arrived inside the most crowded stretch of the whole run, since about 11% of all humans ever born have arrived since 1900. The calculator adds up the cumulative births to your year and tells you how many people had already lived by the time you turned up. Earlier birth years give a smaller number and a rarer position in the sequence, while recent ones sit in the thick of it.

Nobody knows for certain. Stanley Ambrose's 1998 hypothesis in the Journal of Human Evolution tied a population crash around 70,000 years ago to the Toba supervolcanic eruption, but later digs found people in South Africa thriving through it (Smith et al., 2018, Nature), so the Toba link is now disputed. What's firm is that humans carry unusually low genetic diversity next to other great apes, which fits descent from a small ancestral population.

Approximately 385,000 babies are born worldwide every day, about 140 million a year on PRB's figures for 2010 to 2022, and each one adds to the cumulative total. Deaths run well below that, so the population is still rising, though the UN expects growth to stop within this century. Its 2024 projections put the peak at about 10.3 billion in the mid-2080s, up from 8.2 billion in 2024, followed by a slow decline.

Almost certainly no. Double the global population to 16 billion and the living would still be only about 13% of all humans ever born, because the cumulative total keeps growing with every year of births. Getting past 50% would need a sustained population of many tens of billions for centuries while somehow stopping deaths from accumulating. The structure of the arithmetic rules it out.

No. The PRB estimate and this calculator count Homo sapiens only, from a notional starting pair in 190,000 BCE. Add Neanderthals, who lived from roughly 400,000 to 40,000 years ago, along with Homo erectus and the rest, and the total climbs a long way. Population data for those species is extremely speculative, and where one human species ends and another begins is still argued over, so the 117 billion figure is deliberately conservative in scope.