Mitochondrial Haplogroup Z
One group of early mitochondrial haplogroup M individuals broke away in the Central Asian steppes and set out on their own journey following herds of game across vast spans of inhospitable geography. Around 30,000 years ago, the first members of haplogroup Z began moving north into Siberia, the beginnings of a journey that brought them into much of eastern Asia.
Haplogroup Z arose on the high plains of Central Asia between the Caspian Sea and Lake Baikal. It is considered a characteristic Siberian lineage, and today accounts for around 3% of the entire mitochondrial gene pool found there. Because of its old age and frequency throughout northern Eurasia, it is widely accepted that this lineage was carried by the first humans to settle these remote areas.
Radiating out from the Siberian homeland, haplogroup Z-bearing individuals began migrating into the surrounding areas and quickly headed south, making their way into northern and Central Asia. A frequency gradient of Hg Z is observed the further from Siberia one looks: it now comprises around 2% of the people living in East Asia.
Heading west out of Siberia, however, this gradual reduction in frequency comes to an abrupt end around the Ural Mountains and Volga River. This provides a clear example of the impact geographic barriers have on human migration, and thus on gene flow and mixture. To the west of the Urals, it is observed at frequencies less than 1-2%, both in northern and northeastern Europe. Haplogroup Z individuals were quite unsuccessful at making it across these mountains.
Owing to their experience in the harsh Siberian climate, haplogroup Z individuals would have been ideally suited for the arduous crossing of the Bering land bridge during the last glacial maximum, 15,000-20,000 years ago. Colder temperatures and a drier global climate locked much of the world's fresh water at the polar ice caps, making living conditions near impossible for much of the northern hemisphere. But an important result of this glaciation was that eastern Siberia and northwestern Alaska became temporarily connected by a vast ice sheet. It is possible that haplogroup Z-bearing individuals, fishing along the coastline, followed it.
The other Siberian mitochondrial lineages that have similar geographic distributions to Z made it successfully into the Americas. However, haplogroup Z individuals did not, or at least their descendants have not survived to the present day. If in fact Z-bearing individuals were among the first humans to brave the Bering Straight, the number of people carrying your lineage was so low that their contribution to the gene pool in the Americas has been lost.