No matter how much we might like to not think about it the pure fact is that climate affects human populations and climate has always affected human populations since the dawn of time. Sometime around 2.58 mya what is known as the Quaternary Period or System began and it continues into our own present day. This period has been one of many ups and downs in terms of climate with glaciations and interglacials, ice ages and warming periods.
Within the Quaternary Period there is the Pleistocene Epoch which began around 2.58 mya and ended 12.6 kya. Our own epoch is known as the Holocene and it began about 11.7 kya and continues today. During the grand Quaternary Period not only have there been repeated glaciations and interglacials but there have also been what some call “mass extinction events” in which animals and plants became extinct. The reasons for such extinctions are many but one factor always seems to play a part and that is climate change! For example, animals accustomed to living in warm wet climates suddenly begin to go extinct when the climate changes to cold and dry. There reverse is true also in other examples. My point is this. Different forms of life depend on different types of climate and when those climates change it creates chaos and even death in some populations, humans included.
Natural selection was NOT always a matter of what you might call “survival of the fittest” but has also been one of survivors being those most adapted or adaptable to their changing environments. Diverse species emerge in different climates and this includes diverse species of ancient human ancestors. These species developed adaptations to their environment such as erect walking, tool making, prolonged maturation, complex social and psychological behaviors, crop domestication (farming), animal domestication, and more.
When we consider what is known as the “Oxygen Isotope Curve” over the course of the past 10 million years we can see that during the time of Ardipithecus (5.6-4.4 mya) there were increased climate fluctuations and a great deal of environmental uncertainty. Ardipithecus was, in traditional thought, followed by Australopithecus and somewhere inbetween these two species bipedalism evolved as an adaptation to their environment some have theorized. Perhaps it was a response to increased solar radiation. Upright bodies have less exposure to the sun. Perhaps it was a response to being better able to carry things and walk at the same time. Perhaps it was for some other reason but the fact is upright walking began and most likely it was, at least in part, a response to climate change.
With Australopithecus (4.5-2 mya) we see the making and use of the earliest stone tools. Humans had to innovate and develop tools to aid them in their environment which was changing. If they didn’t then none of us would be here today! Following this species came Homo habilis (2.1-1.5 mya) the first of the genus Homo! H habilis is nicknamed “tool man” or “handy man” because this species began to develop better stone tools and to use them for a wider variety of tasks.
Next came Homo erectus (1.9 mya to 70 kya and perhaps even to 30 kya) the great adventurer and explorer! Here we find the first use of fire, cooking, and a new stone tool industry much more sophisticated than anything before. And it is with this species, according to standard evolutionary theory, that we find the first MIGRATIONS OUT OF AFRICA and into the world! These migrations I believe were directly connected to climate change happening at the time. When we look at the Neanderthals (250 kya to 30 kya) we find the first use of clothing which likely was in response to surviving cold climates. Theory has it that as the climate warmed the Neanderthals began to go extinct. Again, a direct response to climate change and in the case of the Neanderthals it was a response to a warming period.
Some changes in climate occur over short periods of time while others take longer. Some are cyclical as in daily or seasonal variations pertaining to such things as temperature, amount of sunlight, precipitation, etc. Large scale climate changes cause large shifts in temperature and precipitation that cause major changes in vegetation amounts and types. Our ancestors had to cope and adapt to such changes to survive. Species of any animal, plant, bird, fish, insect who adapt, survive. Those who didn’t went extinct.
It is believed that Homo erectus populations decreased to somewhere between 600 and 10,000 breeding pairs when the world’s climate was assaulted by mega-droughts, heavy monsoonal rains, and a volcanic eruption near Sumatra about 70 kya. During this time there appears to have been drastic climatic changes in Africa and Eurasia. Further, about 5-10 kya the Earth’s axis of tilt shifted (precession) and this changed the amount of rainfall in Africa among other things! Every few thousand years, we know, the climate in Africa oscillated between wet and dry and each shift between the two brought about adaptations in organisms living there including ancient human ancestors. For example, 3.35 mya in what is now Ethiopia there lived the australopithicines the most famous of which is “Lucy.” There her species thrived for a long time until the climate changed and the environment switched from one of woodland to Savannah. Then around 2.95 mya it switched back to woodland and Lucy’s species was unable to adapt and went extinct somewhere around 900 kya!
As the climate in Africa became one of intense dryness it is believed that it was during this time that Homo erectus began to migrate out of Africa and into Eurasia and Southeast Asia. Dry periods tend to make watering holes far and few between. Dry periods tend to make food and prey scarce! Homo erectus likely was moving out of Africa in search of food and water because their survival depended on it! Prior to this the African environment was wet with plenty of food both plant and animal. Homo erectus had to adapt to the drier environment and his answer was MIGRATION OUT OF AFRICA and into more fertile regions. Climate is not a backdrop! It is a MAJOR factor in human evolution and human survival and STILL IS TODAY!
Over the course of these many climate changes on planet Earth we also have mass extinction events and those mass extinction events ARE tied to CLIMATE CHANGE. In fact both the fossil record and paleoclimate temperature records over the past 529 million years show direct correlations between climate change and these mass extinction events!
One such mass extinction event is known as the Permian Extinction that happened about 251 mya and in which about 70% of the land plants and animals died off along with 84% of the ocean life. We know that this period was an extreme warming period and we also know that such global warming events have been consistently associated with associated extinction events. But it is just NOT warming events associated with extinction events because global cooling also has caused extinction events! For example, at the end of the Ordovician period about 430 mya 60% of various marine organisms went extinct.
I might note that the way we get paleo-temp records is from sediment core samples taken from lake beds and ocean floors.
During to more recent times for a moment let us consider the Mayan civilization of Central America. Between 750-1150 AD (a 400 year span) we know that Mayan population dropped some 70% or more. Some researchers believe this was due to droughts. The same theory has been proposed regarding the people of Chaco Canyon in New Mexico. It has been postulated that the Chaco people abandoned the canyon due to prolonged and repeated droughts and migrated northward possibly to the Mesa Verde area.
Looking at the era of Homo habilis (Handy Man/Tool Man) this species existed at the time that the Pliocene was giving way to the Pleistocene 2.5 mya. Climates in that time tended to be drier and cooler. As the Pleistocene came about it was a period marked by repeated glaciations and interglacials which lasted for tens of thousands of years. During the coldest glaciations global temps dropped about 9 degrees Fahrenheit (5 C) and ice sheets spread out from the poles and high mountains. The Pleistocene had at least 4 or more MAJOR glaciations and inbetween were interglacials (warming periods) with temps fairly close to what they are today.
Below is a chart showing those glaciations and interglacials. Notice that the Pliocene seems to have had WARMER conditions than the Pleistocene.
Below is a chart showing Global Temp Change and Ice Volume over the past 450,000 years. Notice that about 125 kya there was a global change in temperature by about 4 degrees Celsius and about 325 kya there was another spike in warmer temps to the tune of about 3.5 degrees Celsius.
Homo sapiens (modern humans) it is generally believed emerged 200 kya. If we look at the graphs above we see that 200 kya the planet was in a state of transition from a glacial (cooling) to an interglacial (warming) period. We see that ice volume was relatively low and the average global temp was rising.
Climate is far more than just a backdrop. It is often a deciding factor in life events! No matter what side of the fence you are on today with all the debate around global warming one thing is absolutely certain and that is that climate change is REAL and is happening NOW. What this will bring for the future of humans and all other lifeforms on this planet is yet unknown. However, we can be assured that the present climate change WILL have an effect on everything on this planet including us.