When looking at these human species as a whole, we cannot assume that they follow a linear progression, where one species was alive at one time and then a small group evolved into another species. The evolutionary tree of humans, and all species, is just that – it is a tree with different branches that lead from common ancestors to a variety of species, depending on the environment in which the species lives and the mutations that take place as a result of reproduction. Therefore, many species overlapped each other and some even interacted with one another, which is why some modern humans have a bit of Neanderthal in them.

Current research suggests that the genus Homo had their origins in either eastern or southern Africa or possibly in multiple locations on this continent. Findings also point to environmental changes that paved the way for certain groups of primates to become bipedal (those that walk upright, on two legs). Environmental changes such as the proliferation of grassland areas and the loss of trees meant that these primates had to be able to see over the grass to protect themselves from becoming prey. Eventually, those primates that could stand on their hind legs for long periods were the ones that survived in these conditions and over millions of years, this genetic mutation was passed down to future generations. These primates would eventually become our early human ancestors, with the ability to walk upright.

One of the earliest species of the Homo genus was Homo habilis or handyman human. Fossils have dated this species from about 2.4 million years ago to 1.4 million years ago. The Leakeys discovered the first fossils of this type in Africa and they were named habilis for their use of tools.

The Homo erectus species, or upright human, evolved around 2 million years ago and became extinct about 140,000 to 100,000 years ago, making them one of the longest living human species. They also were the first to leave Africa and journey to other areas including the Middle East, Europe, and Asia. While other species of humans existed during this period, the one that most closely resembles modern humans is Homo neanderthalensis. Also known simply as Neanderthals, this species evolved around 500,000 years ago, and became extinct, either through a lack of resources or due to clashes with Homo sapiens or both, between 40,000 and 25,000 years ago. While technology for dating fossils has improved over the years, it can still be difficult for archaeologists to date organic fossil remains. This means that date ranges for evolution and extinction can vary.

Neanderthal fossil remains can be found in both Europe and southwestern and central Asia. Neanderthals lived in colder climates than early Homo sapiens and were therefore heavier set because they needed more body mass to keep warm in cold climates. They were also not as sophisticated as modern humans, as can be proven by comparing their tools, weapons, and burials to those of early Homo sapiens.

Modern humans evolved in Africa approximately 300,000 to 250,000 years ago, and recent studies have shown that the first Homo sapiens may have come from the eastern and/or southern parts of the continent. While other species of humans ventured outside of Africa before the arrival of Homo sapiens, modern humans also began to leave the continent around 80,000 to 60,000 years ago. It is probable that these early humans crossed over into the Middle East (modern day Saudi Arabia) and then hugged the coast and moved into central and south-east Asia. Some of these adventurers then took longer journeys by boat to Indonesia, Papua New Guinea, and Australia by 45,000 years ago, while others moved into Europe through Turkey and the Mediterranean coast about 40,000 years ago, not long before the Neanderthals began to die out. During this period, some mixing of the species occurred through reproduction but ultimately, the Homo sapiens proved to be the more adaptable species.

The traditional time period for modern humans moving into the Americas was approximately 15,000 years ago via the Bering Strait land bridge toward the end of the last ice age. However, new DNA testing has some archaeologists and anthropologists questioning these dates and this area of migration. A group of scientists studied the possible resources available to early humans who might have used the land/ice bridge to cross over into the Americas and found that these migratory people would not have been able to survive the crossing until at least 13,000 years ago. This has led many to speculate that the arrival of early humans into this area must have taken place via the Pacific coast rather than across the land bridge. More research and studies must be completed in order to fully know where and when early humans came into the Americas. However, what is known is that by 15,000 years ago, Homo sapiens had migrated throughout the entirety of the world, apart from the poles and some remote islands.