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How many people have ever existed?
CrowdScience
Apr 5, 2024

Today there are over eight billion people on Earth. That’s an awe-inspiring figure… but how does it compare to the vast numbers who came before us? Listener Alpha wants to know how many people have ever existed, so CrowdScience sets out to do a historical headcount.

The Population Reference Bureau in the USA estimated this number back in the 90s, and have been updating their calculations ever since. Demographer Toshiko Kaneda explains how their model works, the assumptions it makes – and the huge uncertainties around the number it comes out with.

We first need a date for when ‘humans’ first began, so Caroline travels to the Natural History Museum in London to meet human evolution expert Chris Stringer, and marvel at his collection of replica fossil skulls. Chris demonstrates how to distinguish our species, Homo sapiens, from other species like Neanderthals. When did these species first appear - and which of them count as human?

And once you know where to start the clock, how do you estimate the numbers of people alive at different points in history? For a population demographer like Walter Scheidel, it helps that some ancient civilisations kept detailed censuses, a few of which have survived to the present day. Caroline and Walter pour over one of these census fragments, and learn how to combine them with other archaeological clues to get some very rough numbers.

And finally: what does the future of our population look like? Poonam Muttreja from the Population Foundation of India discusses developments in the world’s most populous country, as well as the big demographic trends ahead for humanity.

Presenter: Caroline Steel Producer: Phil Sansom Additional Recording: Umaru Fofana Editor: Cathy Edwards Production Co-ordinator: Connor Morgans Studio Manager: Sue Maillot

Featuring: Toshiko Kaneda, Technical Director of Demographic Research, Population Reference Bureau Chris Stringer, Research Leader in Human Evolution, Natural History Museum London Walter Scheidel, Professor of Classics and History, Stanford University Poonam Muttreja, Executive Director, Population Foundation of India

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Jun 5, 2026
Do plants have personalities?

CrowdScience listener George is showing Alex Lathbridge around a small, dark, and extremely hot shed, just outside the city of Accra in Ghana. Inside are row after row of shelves, stacked high with bulging grow-bags. And out of some of them, gorgeous cascades of oyster mushrooms are bursting into bloom.

We’re on George’s mushroom farm, and he’s noticed something interesting. Even though the conditions in his grow-shed are tightly controlled – they have exactly the same food, water, and light as each other – nevertheless, they respond differently. Some are more vigorous than others, some bloom quicker, others last longer, and some are more tolerant when the conditions change. And this got George wondering. Could ‘brainless’ lifeforms like mushrooms, and plants, have different ‘personalities’? Do they experience the world differently, and live their lives differently from each other? Alex Lathbridge is on the case.

He visits the PGRRI, the Plant Genetic Resources Research Centre, for a quick lesson on genetic variation in the plant world. Plants are all different at the genetic level, and it’s those differences which can result in a tastier fruit, or a hardier crop. But would we call traits like these personality?

In the Minimal Intelligence Lab in the University of Murcia in Spain, Paco Calvo thinks that we absolutely should. He studies plant intelligence, and points Alex to a whole host of examples of plants being smart in ways which might surprise you. Each one is an individual, and if we can only slow down enough to appreciate them properly, we’d be able to understand them better too.

Back in Ghana, Alex meets plant physiologist Dr Acheampong Atta-Boateng, in the beautiful grounds of Aburi Botanical Gardens, to meet some of these plants for himself. And he discovers that there’s a whole world of smart, resilient, and resourceful little organisms in the plant world, full of personality, if you know where to look. Who needs a brain!?

Presenter: Alex Lathbridge

Producer: Emily Knight

Editor: Ben Motley

(Photo: Drawing of a face and smiling eyes on a sunflower flower - stock photo- Credit: Jose A. Bernat Bacete via Getty Images)


29min 16sec


How many people have ever existed?

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