Almost 90,000 years ago, a group of humans in East-Africa decided to undertake a journey which was to change the course of human history. This journey— across substantial geographic and climactic barriers— including deserts, mountain ranges, bodies of water, and glaciers, was one of earliest challenges to be undertaken by the then relatively new species.
By the time this migration came to an end, human beings had settled in all the inhabitable continents around the world. Why did our ancestors embark on such an endeavour? Will we ever see a migration of this magnitude ever again? Let’s look at some of the answers available.
Many scientists believe that humans evolved in Africa. This theory says that humans originated in Africa and then moved on to inhabit the rest of the world. In other words the world was colonised by humans though a series of migrations, which began in Africa.

Where is the evidence?
DNA, the genetic blue print, is present inside the nucleus of every cell of our body. It is passed on from generation to generation. It is the DNA of the cell’s mitochondria which is used to chart the course of evolution. Mitochondrial DNA is inherited only from the mother. This facilitates plotting of a direct line of inheritance.
Investigations of human mitochondrial DNA indicate the relative antiquity of the African people and recency of other populations. This means that all humans can trace their origin back to a common ancestor in Africa. As humans left Africa, some versions of their genes became progressively lost. So the population left behind in Africa retained a high level of genetic variation while the migrants lost most of it.

The first steps
The early migrants moved out of Africa and moved eastwards. On entering what is now Middle-East Asia, the migrants branched out. One set of humans moved into Europe (where they replaced closely-related species called the Neanderthal man), while another continued the journey, entered the Indian subcontinent and spread further eastward into South-East Asia and finally, Australia. Later, North and South America were colonised by migrants from Asia. Luckily for them, the Bering Strait, a relatively shallow body of water between Russia and Alaska, partially dried up, producing a vast stretch of land that united the two continents.
This happened during a period when most of the water on earth was locked up in polar ice caps. Consequently, the sea levels around the world fell and low-lying areas appeared. Similar bridges between other areas may also have facilitated the movement of humans elsewhere.
As each batch of humans colonised a new area, they adapted themselves to the local climate and developed features suited for survival in the local conditions. This is why we find so much variance in skin colour amongst different populations spread across the world. This also accounts for variation in culture and traditions. By the time language was fully developed and the first civilisation appeared on the planet, the migration was almost complete. Therefore, the details of what was undoubtedly the most fascinating journey was never recorded or documented.

Why did they move?
This is an interesting question. If there weren't any sudden and adverse climatic changes, why did the humans migrate? Ecologically speaking, species do move out to new areas once the colonisation of a particular area is completed. By migrating, intra-specie (within species) competition, for resources (like food and shelter), is eliminated. Migration also gives species a greater chance for survival as the risk of extinction, of the entire species, due to natural disasters or even epidemics is eliminated.
So have the humans finally settled down? The answer is a definite no. Migrations continue to this day. Though now we do have personal or professional reasons for moving out of the places where we were born. The coming years however, will see migrations on a larger scale as the full impact of the climate changes fuelled by greenhouse gases start becoming more apparent.
As many low-lying islands and coastal areas get flooded due to rising sea levels, the new migrants will commence their journey with even more uncertainty than their ancient counterparts. Unfortunately, unlike their ancestors, they will not have much land to move on to. Even if we manage to survive the effects of global warming, we may still have to cope with the sun running out of fuel.
When this happens, the sun will turn into a red giant and swallow up planets like Mercury and Venus. This event will happen in another 5 billion years from now. By then, scientists say that we would have developed the ability to travel between galaxies. This event would then trigger the biggest migration ever, as humans look to colonise worlds outside the solar system or even the Milky Way.

Some facts
*Most of the development that we have seen till now happened over the last 5000 generations
*Some ancient tools found in North America are entirely similar to those found in China
*Genetically speaking, there is more variation among some groups of Chimpanzees than among humans across the world.
*What happened to the Neanderthal man is still unclear.