READ: Human Communities Populate the Earth

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Try to imagine yourself living 15,000 years in the past. You are a forager (hunter-gatherer) trekking across great distances in search of food. Now imagine how groups of foragers might have lived as they moved into these new territories. There aren't any cars or phones or even wheels at this stage of history. So how did people come to inhabit almost all areas of the world by about 15,000 years ago? And how do we know about their movements and their way of life?

Our current knowledge of human origins and migration patterns has been aided by the research of scholars in a number of different fields: archaeologists, anthropologists, climatologists, geneticists, historians, linguists, and paleobotanists. The vast majority of scholars in these fields would agree that modern humans, Homo sapiens, evolved in Africa approximately 300,000 to 200,000 years ago. But we weren't the only humans around during that time. Other human communities composed of different species such as Homo neanderthalensis (Neanderthals), Homo erectus, and Homo floresiensis existed in various regions of Afro-Eurasia. But Homo sapiens, humans like you and everyone else in the world, are the only human species that are still alive today. Homo sapiens have a slighter build than earlier human species and have a distinctly shaped skull, as can be seen in the image below comparing a Homo sapiens skull and a Neanderthal skull. Evidence of our earliest existence comes from fossils found in Ethiopia about 200,000 years ago. But some researchers think that our species was around for longer than this.

Debate continues around whether Homo sapiens encountered all of these species as they migrated out of Africa approximately 80,000 to 60,000 years ago. These migrations probably occurred at a range of times and to a variety of places rather than just one large migration out of Africa starting at one specific time period. In fact, new evidence suggests that the earliest date for these migrations may be 120,000 years ago. There is evidence that Homo sapiens interacted and reproduced with Neanderthals, which are another species of humans. Neanderthals mainly lived in colder climates, had a heavier build, and their foreheads were more sloped with a more pronounced ridge at the front. The result of these interactions is why some modern humans have small percentages of Neanderthal DNA. In addition, new studies show that some Homo sapiens mixed with both Neanderthal and another human species known as Denisovans in Central Asia and Australia (Bae, et al). Denisovans are a more recently discovered human species that mainly lived in the area of modern-day Siberia. They interacted and reproduced with Neanderthals, who also lived in these colder regions.

While a definitive date hasn't been decided, we do know that communities of Homo sapiens began journeying outside of Africa thousands of years ago. Eventually, other human species died out. This could have been a result of mixing with Homo sapiens communities, through violent clashes with modern humans, or due to other effects such as climate and environmental changes. All of these early human communities were nomadic foragers, or people who moved around to follow their food. Foragers lived off the land by gathering, hunting, and fishing. Archaeologists and anthropologists have suggested that the number of people in each community was likely kept rather small, no more than 20-50 people. These humans primarily used stone tools along with others made of bone. For this reason, archaeologists and historians have categorized the period as the Paleolithic era (Old Stone Age). Population numbers probably remained relatively small throughout the Paleolithic era. These human communities may have intentionally kept their population growth rates low in order to be able to move more easily. It would be very difficult to carry multiple children at one time to gather, hunt, and move to follow your food.

These small groups collected food and resources from one area and then moved to another as food became scarce. They interacted with other communities and established networks of exchange to trade items they gathered or made. In addition, language networks connected people who lived in the same general areas so that information was shared amongst groups. Some members of one community left to join others in order to have children and establish new family groups. Larger networks of people also came together for spiritual or religious purposes.

Over time, some groups migrated outside of Africa. These movements were possibly as a result of climate changes and scarcity of resources, or human curiosity, or conflicts with other groups, or some combination of these causes.

Humans began moving perhaps as early as 120,000 years ago and probably in waves that occurred to different regions at different time periods. The earliest migrations might have occurred along the coasts of the Arabian Peninsula into India and then on to Southeast Asia. These groups stayed near the coastline because there was an abundance of resources such as marine life to sustain them. Early humans constructed rafts or small dugout boats to maneuver across larger expanses of water. By about 50,000 to 45,000 years ago, these communities reached Papua New Guinea, Australia, and East Asia. Recent evidence suggests that humans' arrival in Australia may be much earlier than this, with some studies pushing the date back to 65,000 years ago (Clarkson et al., 2017).

Evidence also shows that other groups left Africa and took a more northerly route through the Arabian Peninsula and entered Europe and Central Asia by about 45,000 years ago. Some communities that had migrated and adapted to much colder climates in northern Asia journeyed across the Bering Strait by as early as 20,000 years ago. Small groups may have then traveled along the coasts and made their way into the Americas from about 15,000 years ago. Still others may have waited until the climate became warmer to journey by land into the Americas about 12,000 to 10,000 years ago. Regardless of the route taken, early humans made their way down to the southern tip of South America by about 15,000 to 10,000 years ago. The fast pace of migration through the Americas was the result of an abundance of resources both from marine life and large game animals. Many of these large game animals that were living in the Americas were eventually wiped out as humans moved across the continents.

As human communities migrated, the environment affected not only how these humans lived but also how they looked. Humans adapted to new environmental conditions as they moved around and populated the Earth. One reason they achieved success living in varied climates was the result of collective learning, or the sharing and preserving of information across generations. As humans learned new ways of dealing with their environment, they shared this information with others in their communities. These tools and techniques for survival were then passed down from generation to generation, allowing humans to become the first species to populate the entire Earth. Collective learning is, therefore, what allowed our human ancestors to populate the Earth. By sharing and preserving knowledge across generations, humans made significant improvements to how they lived and survived.

But as humans moved, they changed. These changes weren't drastic but there were slight, random mutations to DNA that occurred as humans lived in certain areas for longer periods of time. These random mutations account for the differences that you see in humans today. We are all of the same species but have slightly different appearances such as skin and hair color, or the shape of our eyes and noses. For example, humans who migrated to colder climates needed more vitamin D from the Sun rather than more protection from the Sun that humans living near the equator needed. Over thousands of years, these humans experienced random genetic mutations that led to some advantages when adapting to their new environments such as the lightening of skin color (through a decrease in melanin) to allow more of the Sun's rays to penetrate skin and provide more of this necessary vitamin. In fact, this process was happening before modern humans ever left Africa (Crawford, et al. 2017).

Humans also impacted their environments as they migrated. They hunted animals, some to the point of extinction. They set fire to wooded areas in order to drive out wildlife and more easily hunt. Their movements impacted the environment and continue to do so in different ways today.

The story of the origins of humans and their migration patterns out of Africa are in an almost constant state of flux. This is a result of collective learning at work as scholars continually make new discoveries that lead to revisions in the history of humanity. We know broadly the time, place, and manner of movements but the specifics are still being identified. As more fossils, tools, and genetic evidence are discovered, the story of early human communities may change multiple times. But through academic discussions and validation of evidence using multiple disciplines and sources, the story of our ancient past may become clearer in the very near future.

Bridgette Byrd O’Connor holds a DPhil in history from the University of Oxford and has taught Big History, World History, and AP U.S. Government and Politics for the past ten years at the high school level. In addition, she has been a freelance writer and editor for the Big History Project and the Crash Course World History and U.S. History curriculums.