WHERE DO WE COME FROM? MORE RECENT THEORIES

In the previous blog post I talked about the Savanna theory which, for a very long time, was the prevailing concept in the field of human evolution. However, more complete paleontological evidence revealed the complexity of our evolutionary journey, as well as gaps in our knowledge. In today’s post I will focus on more recent, more complex hypotheses of our origins.

In the 1980s, Elisabeth Vrba developed the turnover pulse hypothesis which expanded on already established themes (climate as a driver of evolution and an increasing aridity in during the Pliocene) while also challenging the rates of change (Kingston: 2007). In a nutshell, Vrba initially stated that speciation and extinction events were concentrated in a short period of time (also called ‘the 2.5 million year event’) due to a shift towards much drier conditions. However, the developments in paleoclimatology exposed the shortcomings of this hypothesis: current evidence revealed periods of extreme climatic variability in East Africa during the Plio-Pleistocene, which made the assumption of a smooth transition between wetter and drier conditions rather invalid. Besides, fossil evidence indicates more than one speciation event.

In the light of new evidence, Rick Potts developed the variability selection hypothesis in the 1990s. He said that the key events in human evolution resulted from increasing environmental instability rather than just a single environmental trend and change of habitat (Human Origins Program: 2014). So, this hypothesis states that hominin evolution wasn’t an adaptation to the drier climate and encroaching savanna, but to climatic variability. It argues that because of the environmental fluctuations and habitat fragmentation, habitat-specific adaptations were replaced by adaptations for versatility, such as bipedality and brain expansion in hominins.

One of the most recent ideas of human evolution is the pulsed climate variability hypothesis, developed by Mark Maslin and Susanne Shultz (Shultz and Maslin: 2013). This hypothesis is largely based on the evidence from ephemeral East African paleolakes which revealed extreme wet-dry climate cycles in the region during Plio-Pleistocene. The hypothesis suggests that major events in hominin evolution coincided with the presence of deep lakes in the region. It supports the view of climate variability being an important evolutionary driver; however, it states that climatic pulses, not a long term trend towards increased variability, drove speciation and subsequent migration events. The relationship between climatic pulses and hominin evolution is not that simple though; Maslin and Shultz mention that a significant brain expansion event, which happened ~1.8 Ma, concurred with a very wet phase in East Africa, while following expansions happened during periods of extreme aridity.

There are many more hypotheses that explain hominin evolution – I have just outlined a few. Despite increasingly complete evidence, there is still no real consensus for what drove the change.  In my view, it is very likely that climate wasn’t always the major evolutionary force and that different human features evolved through different mechanisms. The answer might lie in combining different hypotheses and acknowledging the complexities of evolution instead of trying to simplify the process and limit ourselves to one straightforward answer.

What can we take from this evolutionary discussion then? Well, on one hand we might feel relieved because if we are a result of adaptation to climatic variability, if we were ‘born from climate change’ (as stated by Mark Maslin), then we shouldn’t fear the current alterations in climate. On the other hand, we are the only Homo species left from what was once a diverse family tree which means that being adaptable does not guarantee survival. Plus, current climatic changes happen at a much quicker rate than Plio-pleistocene ‘extreme’ events. Rick Potts said:  "In the long view, the line between thriving and decline is a fine one. That is a theme of human evolutionary history" - this is what I will be exploring throughout my future blog posts.

WHERE DO WE COME FROM? THE SAVANNA THEORY

In one of my previous posts I mentioned that the records from East African paleolakes revealed huge climate variability in the region which coincided with major events in human evolution. I also talked about how this discovery changed the understating of our origins and created more questions than it answered, giving a rise to numerous theories explaining our adaptation.

Before exploring these new theories though, we need to know what our understanding of early human evolution was previously. The most popular theory was the Savanna theory which was ‘officially’ developed by Raymond Dart in 1925, but it actually started with Lamarck in 1809 and Darwin in 1871, and was still widely approved in the 1980s (Bender et al.: 2012). It was founded on the assumption that the last common ancestor of humans and great apes lived in forests, but when the global climate started cooling down and African uplift and rifting took place, the forests were replaced by patches of open woodland and grassland. According to that theory, our current form is a result of adaptations to the new habitats. As the resources became more sparsely distributed, bipedality enabled carrying of food and water over long distances. The Savannah theory also explains the hominid brain expansion by food scarcity and the necessity of complex social structure for survival (Potts: 1998). As a consequence, large brains needed more fuel so, instead of eating more plants, our ancestors turned to meat. Our bipedalism makes us one of the best long-distance runners in the animal kingdom, and little hair allows sweating and facilitates cooling. These features were adaptations to chasing animals in vast open spaces, under the sizzling African sun with no or little shade due to the lack of forests. Our advantage was not speed or strength, but the ability to run after an animal to the point it became exhausted and collapsed.     

However, recent discoveries showed us that the idea of simple transition from ape to human from is not quite right – things turned out to be way more complex than that. In the next post I will focus on some more recent, more complicated hypotheses of our origins and I will also present limits to our current knowledge.

A simplified illustration of the Savanna theory (from: toknowthyself.files.wordpress.com)

Changing climate and human evolution on the news!

A great start to a Saturday - I came across some interesting news this morning! New discovery shows evidence of a genetic ‘unity’ between the first modern humans in Europe and later peoples. The discovery challenges the current assumption that Palaeolithic hunter-gatherers went extinct during the last Ice Age. On the contrary - it suggests that some of them managed to survive the Ice Age and colonise the landmass of Europe for more than 30,000 years. Click here for details!

EAST AFRICA - THE CRADLE OF HUMANKIND

In this post I will discuss the influence of climate change on our origins. The first question we might ask ourselves here is: when and where did it all happen? And then, naturally, a more complicated question occurs: why did it happen? The answers are quite complex - scientists have spent decades exploring them – but here I will summarise the most important findings.
 

When and where?

Existing evidence suggests that the majority of hominin species originated in East Africa, and that’s why we will look closely at the climatic variations in that particular region (Maslin et al.: 2014). (In this place it’s worth noting that by hominin I mean all bipedal apes, as opposed to hominid which refers to all great apes). The question of the timing of our beginning is a bit trickier – I will treat the emergence of be the first Australopithecus species, Australopithecus afarensis, as the starting point. The figure below explains why.

Anatomical comparisons of apes, early hominins, Australopithecus, Homo erectus, and humans. Nature Education: 2012 (www.nature.com/scitable/knowledge/library/overview-of-hominin-evolution-89010983)

Early hominin evolution coincided with both global cooling and extensive tectonic changes in East Africa (Behrensmeyer:  2006). In very simple words, marine-core records (as well as long cores from deep African lakes, but we’re looking at a global picture now) indicate that a colder, drier and – very importantly – more variable climate begun 4-3.5 million years ago, triggering the climatic shift towards northern continental glacial-interglacial cycles (Trauth et al.: 2007). But, although the fossils tell us a lot about what happened and when; they can’t tell us why – it’s all down to our interpretation of the records.

Why?

There are a few factors that lead to said climatic shift and, consequently, environmental and evolutionary changes. There is no real consensus on the reasons behind the late Cenozoic intensification of Northern Hemisphere Glaciation. Some attribute it to tectonic changes, such as the uplift and erosion of the Tibetan-Himalayan plateau, the restriction of the Indonesia seaway or the emergence of the Panama Isthmus. Others relate it to the long-term decrease in atmospheric CO2. However, whatever the reasons behind it, the intensification of Northen Hemisphere Glaciation resulted in increased aridity in East Africa (Hetherington: 2012).

What’s more, such unfolding of events in high latitudes lead to the compression of the Intertropical Convergence Zone, which consequently rendered East Africa locally sensitive to orbital (precessional) forcing. In other words, it brought about extreme climate variability and the East African overall long-term drying trend was punctuated with periods of wetness.

Rapidly changing condition and dramatic variations in climate obviously had a huge influence on the environment, and as I said at the very beginning of this blog, our environment is what shapes us. No wonder many scientists believe that extreme climate variability in East Africa might have been a catalyst for evolutionary change and hominin speciation.

In the next posts I will go into detail of environmental effects of climatic changes I talked about, and I will explain their possible influence on the development of hominin traits.

For more details check:

• Behrensmeyer, A.K. (2006) ‘Climate Change and Human Evolution’, Science, 311, 5760, 476-478
• Hetherington, R. (2012) Living in a dangerous climate: climate change and human evolution, Cambridge: Cambridge University Press
• Maslin, M. A., C. M. Brierley, A. M. Milner, S. Shultz, M. H. Trauth and K. E. Wilson (2014) ‘East African climate pulses and early human evolution’, Quaternary Science Review, 101, 1-17
• Trauth, M. H., M. A. Maslin, A. L. Deino, M. R. Strecker, A. G. N. Bergner and A. Duhnforth (2007) ‘High- and low-latitude forcing of Plio-Pleistocene East African climate and human evolution’, Journal of Human Evolution, 53, 475-486