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.