Humaity possesses inherent value, but we are devastating the Earth and causing unimaginable animal suffering.

There is no other creature in nature whose predatory behavior is remotely as deep or as widespread as the behavior we display toward our fellow creatures.

The Medea hypothesis. The name is a reference to the Gaia hypothesis, named for the Greek goddess of the Earth, which suggests that life helps keep Earth habitable.

Medea, in stark contrast, is a Greek mythological figure famous for killing her own children.

Life’s biggest threat could come from within. Many of the mass extinctions in Earth‘s history were caused by life.

Humanity has come startlingly close to destroying itself in the years in which it has had the technological power to do so.

A nuclear war could be an existential catastrophe. It might be – a nuclear winter could lead to sufficiently dreadful collapse in agriculture to kill everyone – but it seems unlikely, given our understanding of physics and biology.

Climate change, although it may well lead to some truly dreadful outcomes, also is unlikely to render the planet uninhabitable.

In both scenarios, it is very likely that history repeats itself and Kain will kill Abel, only to realize that Eve has eaten from the poisened red apple in the garden of Eden and is dying, being the last fertile woman on Earth.

All things must pass. That includes life on Earth.

We could live for a billion years on this planet, or billions more on millions of other planets, if we manage to avoid blowing ourselves up in the next century or so.

Modern Homo sapiens (people who were roughly like we are now) first walked the Earth about 50,000 years ago. Since then, more than 108 billion members of our species have ever been born.

Given the current global population of about 7.5 billion means those of us currently alive represent about 7% of the total number of Humans who have ever lived.

Fixing a time when the Human race actually came into existence is not straightforward. Hominids walked the Earth as early as several million years ago.

And various ancestors of Homo sapiens appeared at least as early as 700,000 B.C. Modern Homo sapiens may have appeared about 50,000 B.C.

At the dawn of agriculture, about 8,000 B.C., the World population was somewhere on the order of 5 million.

The slow population growth over the 8,000-year period – from an estimated 5 million to 300 million in 1 C.E. – results in a very low growth rate of only 0.0512% per year.

It is difficult to come up with an average World population size over this period. In all likelihood, Human populations in different regions grew or declined in response to famines, the vagaries of animal herds, hostilities, and changing weather and climatic conditions.

In any case, life was short. Life expectancy at birth probably averaged only about 10 years for most of Human history.

Estimates of average life expectancy in Iron Age France (from 800 B.C. to about 100 C.E.) have been put at only 10 or 12 years.

Under these conditions, the birth rate would have to be about 80 live births per 1,000 people just for the species to survive.

By 1 C.E., the World may have held about 300 million people. One estimate of the population of the Roman Empire, spanning Spain to Asia Minor, in 14 C.E., is 45 million.

Other historians, however, set the figure twice as high, suggesting how imprecise population estimates of early historical periods can be.

By 1650, the World’s population rose to about 500 million. The average annual rate of growth was actually lower in this period than the rate suggested for the preceding period from 8,000 B.C. to 1 C.E.

One reason for the abnormally slow growth was the Black Death. This dreaded plague was not limited to 14th-century Europe, but may have begun in western Asia about 542 C.E., and spread from there.

Experts believe that half the Byzantine Empire was destroyed by plague in the 6th century, a total of 100 million deaths.

Such large fluctuations in population size over long periods greatly compound the difficulty of estimating the number of people who have ever lived.

By 1800, however, the World population passed the 1 billion mark, and it has since continued to grow to its current 7.5 billion.

This growth is driven in large part by advances in medicine and nutrition that lowered death rates, allowing more people to live into their reproductive years.