How Earth's population exploded

Since the emergence of the first humans in Africa over two million years ago the world's population has ballooned, with only fleeting pauses to the increasing number of people sharing planet Earth.

As the global population teeters on eight billion -- a milestone expected to be reached in mid-November -- here is a look at the main chapters in the growth of humanity.

First humans

The oldest fossils from the earliest known humans date back 2.8 million years and were found in east Africa.

But estimates of the number of people that populated the earth remained highly unreliable until the 19th century.

What we do know is that our ancestors were hunter-gatherers, who had few children compared to later settled populations in order to maintain their nomadic lifestyle.

The planet's population was sparse also partly because hunter-gatherers needed a lot of land to feed themselves -- around 10 square kilometres per person, according to Herve Le Bras, a researcher at the French Institute for Demographic Studies (INED).

The globe's population did increase over time but very, very slowly.

First baby boom

The introduction of agriculture in the Neolithic era, around 10,000 BC, brought the first known major population leap.

With agriculture came sedentarisation and the ability to store food, which caused birth rates to soar.

"Mothers were able to feed infants gruel, which sped up the weaning process and reduced the amount of time between births, meaning more children per woman," Le Bras explained.

The development of permanent settlements also brought hazards, however, with the domestication of animals causing humans to contract new deadly diseases. 

Child mortality rates were particularly high, with a third of all children dying before their first birthday, and another third before they turned 18.

"There was huge mortality but also a permanent baby boom," Eric Crubezy, anthropologist at the University of Toulouse in France, explained.

From around six million in 10,000 BC, the global population leapt to 100 million in 2,000 BC and then to 250 million in the first century AD, according to INED estimates.

Black Death

The Black Death brought the population to a sudden halt in the Middle Ages.

The pandemic, which emerged in Central Asia, in what is modern-day Kyrgyzstan, reached Europe in 1346 on ships carrying goods from the Black Sea.

In just eight years, it wiped out up to 60 percent of the populations of Europe, the Middle East and North Africa.

As a result of the Black Death, the human population dropped between 1300 and 1400, from 429 to 374 million.

Other events, like the Plague of Justinian, which hit the Mediterranean over two centuries from 541-767, and the wars of the early Middle Ages in western Europe, also caused temporary dips in the numbers of humans on Earth.

Eight billion, and counting

From the 19th century on, the population began to explode, due largely to the development of modern medicine and the industrialisation of agriculture, which boosted global food supplies.

Since 1800, the world's population has jumped eight-fold, from an estimated one billion to eight billion.

For Crubezy, the development of vaccines was key, with the smallpox jab particularly helping zap one of history's biggest killers. 

The 1970s and 1980s brought another small revolution, in the form of treatment for heart disease, which helped reduce mortality among over-60s.