The earliest species of the Homo genus appeared around 2 million to 1. Current evidence supports modern Homo sapiens appearing around , B. Modern Homo sapiens originated in Africa, though the exact location has long been debated. Diverse groups are thought to have lived in different locations across Africa for the first two-thirds of human history. Around B. Table 1 displays very rough figures representing averages of an estimate of ranges given by the United Nations and other sources.
Slow population growth over the 8,year period—from an estimated 5 million in B. In all likelihood, human populations in different regions grew or declined in response to food availability, the variability of animal herds, periods of peace or hostility, and changing weather and climatic conditions. Poston Jr. In any case, life was short.
Life expectancy at birth probably averaged only about 10 years for most of human history. Average life expectancy in Iron Age France from B. Under these conditions, the birth rate would have to be about 80 live births per 1, people just for the species to survive. To put that in perspective, a high birth rate today is about 35 to 45 live births per 1, population, and it is observed in only some sub-Saharan African countries.
These short life expectancies mean that the human population had a hard time increasing. Other historians, however, set the figure twice as high, suggesting how imprecise population estimates of early historical periods can be.
The average annual rate of growth was actually lower in this period than the rate suggested for B. One reason for the unusually slower growth was the Black Death. This dreaded plague was not limited to 14th-century Europe but may have begun in western Asia in about C. Experts believe that half the Byzantine Empire was destroyed by plague in the sixth century, a total of 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 , however, the world population passed the 1 billion mark and has since continued to grow to its current 7. This growth is driven in large part by advances in public health, medicine, and nutrition that have lowered death rates, allowing more people to live far into their reproductive years.
Guesstimating the number of people ever born requires determining population sizes for different points in human prehistory and history and applying assumed birth rates to each period. We start at the very, very beginning—with just two people a minimalist approach! Although it is unlikely that humans descended from two people, this approach simplifies our estimation.
One complicating factor is the pattern of population growth. Did it rise to some level and then fluctuate wildly in response to famines and changes in climate? Or did it grow at a constant rate? We cannot know the answers to these questions, although paleontologists have produced a variety of theories. For the purposes of this exercise, we assumed a constant growth rate applied to each period up to modern times. First you need to assemble a header.
Obviously, the formula is not just used for that and can be applied for different purposes, from knowing how many days have passed since a certain date, or how long a work has been in progress or how many days it has been since you started your job and etc.
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