THE RESILIENT SPECIES

Today’s post will be the last one in my series about climate change and human evolution, and that’s why I would like to share with you some of the thoughts I developed over the past months.

The reason why we study the past so meticulously is not only to satisfy our insatiable curiosity – we also want to be able to learn from the history in order to understand or predict the future. So, what have we learned from this blog that can be applied for the future, especially under a changing climate?

On one hand, I showed that our species was ‘born’ from climate change: our two main characteristics, bipedalism and a large, complex brain were adaptations to environmental variability. Thanks to these features we have become extremely adaptable and we were able to migrate, settle in all kinds of environments and endure numerous climatic fluctuations. On the other hand, I showed that such flexibility has not always guaranteed survival – all but one of the Homo lineage vanished, and a climatic factor is likely to have played a role in some of those extinction events.

However, I haven’t mentioned a very important fact about the relationship between people and climate. Thanks to our intelligence and creativity we became the only species able to alter the environment – including climate – to such an extent. In all my previous posts I just wrote about the effects of climatic changes on our species, but I haven’t explored how our species influences the climate.

The human impact on the Earth’s climate is an unprecedented phenomenon and, let’s face it, we’re just starting to understand it. That is why it is incredibly hard to predict what is going to happen to us in the future. All the lessons from the past might become invalid when confronted with the unparalleled rate of climate change and erratic human behaviour. However, there is no doubt that never before have we had as much power to shape the environment we live in. So, are we going to make the Earth uninhabitable and doom ourselves to extinction, or are we going to use our knowledge, adaptability and imagination to continue being the dominant, most resilient species?

HOMO SAPIENS - THE ONLY SURVIVOR

In my previous posts, I briefly mentioned the once incredible diversity of the human family tree and I looked at the possible reasons behind the disappearance of the Neanderthals. But what about all the other hominin species that did not make it, although at some time in the past they were thriving? Searching for answers is extremely hard. Entire populations who lived for thousands of years left behind only a few incomplete fossils on which we base our speculations.

As an example let’s take Homo florensiensis – also nicknamed ‘the hobbit’.  Homo florensiensis lived on the island of Flores in Indonesia, measured a little bit more than a metre in height and a brain of a size of an orange (Brown: 2012). There are numerous theories on the emergence of the species, but I want to focus on its disappearance which took place approximately 12,000 years ago. There are numerous theories on the emergence of the species, but I want to focus on its abrupt disappearance which took place very recently - approximately 12,000 years ago. One of the popular theories states that H. florensiensis (as well as Stegodon, the pygmy elephant which constituted the main source of food for the hobbit) went extinct because of a volcanic eruption at ca. 12,000 years ago. Some scientists say it could have been the arrival of Homo sapiens that put an end to the millennia of H. florensiensis presence on the island. However, there is some evidence suggesting that changes in climate could have played their part too. The stalagmite records from Indonesia show a sharp increase in rainfall, following a long arid period and coinciding with the H. florensiensis extinction (Westaway et al.: 2007). Unfortunately, due to the lack of complete evidence it is still impossible to tell for sure which of the possible causes had a major part in the vanishing of the hobbit.

With the demise of H. florensiensis, the diversity of Homo lineage finished. We are not entirely certain why H. habilis, H. erectus and other species of the genus Homo disappeared. Environmental stress? Competition?  Interbreeding? Climate change? Or combination of all the factors? Due to that uncertainty, in my next post I would like to focus not on the reasons for extinction of the other species, but on the factors making Homo sapiens the only survivor of the Homo family. I will also look at what the story of our success can mean for our future under the rapidly changing climate.

HOMININ MIGRATION - A RESPONSE TO CLIMATE CHANGE

Up to now, I have talked about numerous ways in which climatic variations affected human evolution, often deciding about a species’ survival or disappearance. However, I have not yet mentioned a very important consequence of climate change: migration. In today’s post I will explore all the complex migratory processes triggered by a changing climate.

Migration has been a typical behavioural reaction to changing climate since early days of hominin history – hominins (namely Homo erectus) first dispersed from Africa approximately 2 million years ago (Aguirre and Carbonell: 2001). Most commonly, hominin species followed flora and fauna they depended upon, and the extent of said flora and fauna shrunk and expanded with long- and short-term variations in climate. However, movements resulting just from habitat expansion did not need any major adaptation. Evolutionarily significant migrations happened when hominins dispersed to new habitats they had to adjust to.

So, let’s go back to arguably the most important migration in our history – H. erectus going out of Africa towards Asia ~2 Ma. That movement coincided with the increase in climatic variability and the gradual shift towards cooler and drier conditions in Africa I talked about quite a lot in my previous posts. Some scientists suggest that it was the changing climatic conditions that shaped the H. erectus morphology that allowed the migration, in particular longer lower limbs that permitted long distance locomotion, but also larger body size, linear body shape and nasal morphology (Eng: 1998). In other words, first the climate triggered the hominin adaptations to arid conditions, and then the adaptations proved to be useful for long-distance migration. And then, the first hominin dispersion led to the numerous speciation events of the hominin species in different regions of the world.    

Climatic variations could have added or removed some of the physical barriers, such as deserts or ice landmasses, for people moving because of resource depletion or population growth. For example, hominin expansion to the tropics occurred during cold, glacial intervals, when the sea levels were significantly lower (Hetherington and Reid: 2010). Also, interestingly, it is likely that genetic exchange between hominin populations increased during glacial intervals as people probably gathered in refugia and new paths opened due to a lower sea level (Antón: 2002, Keates: 2004). On the other hand, the migration towards higher latitudes took place during warm, interglacial intervals.

As usual, the migratory and evolutionary processes and their relation to climate change might be much more complex than we now understand. However, the story of hominin migration as a response to environmental change teaches us a very important lesson: hominin species thrived due to their intelligence, ability to move over large distances and adapt to new territories. Migration is, after all, more efficient and reasonable response to a changing climate than the gradual  process of natural selection and genetic change. 

WHAT HAPPENED TO THE NEANDERTHALS?

Until now, I focused on how changes in climate let us develop the most human of our features: walking upright and big brains. However, I also mentioned that hominins used to be very diverse – there were numerous species living on Earth at the same time. Right now Homo sapiens is the only one present. What happened?

In today’s post I will focus on a very recent example of hominin extinction - the Neanderthals. Homo neanderthalensis was a successful species for millennia - it first appeared in the fossil record about 400,000 years ago and vanished approximately 30,000 years ago, although the exact dates are still disputed (Hublin: 2009). Unsurprisingly, the debate about who the Neanderthals were, how they lived and why they went extinct is also still open.

For a very long time the scientists thought that the Neanderthals vanished because they were outcompeted by modern humans, the Homo sapiens. Modern humans are believed to have arrived to Europe, the stronghold of the Neanderthals, about 40,000-43,000 years ago, and according to this theory they competed with the Neanderthals over resources and territory (Hetherington: 2012). Homo sapiens had several advantages over Homo neanderthalsis. For example, despite many anatomical similarities, their body build allowed them to move faster. Another important advantage was modern human’s brain, which - although smaller on average – was more developed and permitted modern humans better communication or social organisation, crucial to obtaining resources and securing territories. Some other theories claim that migrating Homo sapiens brought certain pathogens to Europe which were unknown to the Neanderthals’ immune system.

One of the most recent hypotheses states that actually the Neanderthal’s extinction as such never took place. There is evidence that modern humans and the Neanderthals interbred; moreover, recent research shows that “between one and four percent of the DNA of many humans living today originate from the Neanderthals” (Max Planck Society: 2010). This might indicate that the Neanderthals did not vanish completely, but might have been “absorbed” by the dominant modern humans (Villa and Roebroeks: 2014).

However, none of the above theories mention the climatic changes that took place at the time of the Neanderthal disappearance. The Neanderthals were a very adaptable species – they were present during both glacials and interglacials. They used to inhabit south-west Eurasia and their habitat ranged from temperate woodland to tundra. They had more body hair and fat than modern humans which made it possible for them to survive in cold conditions. But new evidence shows that when the Neanderthals started vanishing, the climate was particularly harsh, and that their disappearance occurred at different times in different regions, pushing them increasingly further south. Cold and dry weather wiped out most of the Neanderthals sources of food and turned their usual habitats into vast, open spaces that they could not adapt to. Their anatomy made them very successful in wooded areas or steppes – they could, for instance, sneak up on a prey and kill it form close proximity. They could not, however, chase the prey over long distances or throw a spear from afar. Their bodies simply did not allow it. Studies lead by Professor Tom Higham from the University of Oxford indicate that when the modern humans arrived to Europe, the Neanderthals were already in decline and the new competition was just a final blow to the declining population (Higham: 2014). They also suggest that the Neanderthals and modern humans coexisted for up to 5000 years and that the disappearance of Homo neanderthalsis was a gradual process, dispelling the theories I talked about at the beginning of the post.

As usual, it is impossible to find definite answers to all the questions we might have about our very close relatives: our dating methods aren’t accurate enough, and the fossil and proxy evidence records are still far from complete. However, it is very important to continue the research: our DNA differs from the Neanderthals by only 0.12% and we can learn some important lessons from the story of their disappearance. The latest evidence suggest that, although we are such an unbelievably adaptable species, climate change might pose one of the gravest threats to our survival. 

BECOMING HUMAN: BRAIN EXPANSION

We already established that the evolution of bipedalism was the first step towards what we perceive as being human. However, there was another process that was absolutely crucial to making the human species: encephalization, or the evolutionary brain expansion relative to body size (Hofman: 2014). This development resulted in one of the most complex and efficient structures in the animal kingdom: the human brain.

During the first 4 million years of hominin evolution, the brain growth was rather slow. More pronounced enlargement started 2.5 million years ago, most likely from a bipedal Australopithecine whose brain size was similar to that of a modern chimpanzee. The dramatic increase in the hominin brain size happened only in the last 800,000 years. Over the course of approximately 7 million years, the human brain tripled in size (Robson and Wood: 2008).  In this blog post I will try to explain what pressures could have led to brain growth, and what effects it had on the survival of our species.

When I talked about bipedalism, I said that for a very long time the leading theory of hominin evolution went more or less like that: walking upright freed our hands, free hands led to tool use and tool use resulted in a rapid growth in brain size. I also explained how we learned that this theory is unlikely. Nowadays the increase in hominin brain size is explained in two ways: through environmental or social factors.

Pressures of the physical environment

Some of the theories explaining the increase in brain size focus on the environmental selection pressures, especially climate. The rapid encephalization in hominins coincided with the period of particularly large climatic fluctuations, as shown in the figure below.

Source: http://humanorigins.si.edu/research/climate-research/effects

In order to deal with such unpredictability, our ancestors had to think ahead and develop much more sophisticated cognitive abilities in order to adapt. Some scientists say that the climate variability led to a diversification of our diet – in order to survive our ancestors had to become omnivores and come up with innovative ways of obtaining food (Willemet: 2013). Moreover, the addition of meat to their diet was a very important source of energy, much needed for the growth of brain (right now our brain is approx. 2% of our body weight, but requires at least 20% of our calorie intake to function).

Pressures of the social environment

There are also hypotheses about social factors leading to brain expansion in hominins. Population growth could have been an important factor, and it could have affected brain size in various ways. There could have been a competition for resources and consequently smarter, more innovative individuals would have better chances of survival (Falk: 1990). Alternatively, due to an increase in population our ancestors could have started to form larger, more complex social groups which involved cooperation, coalition formation or reciprocal altruism – all requiring intelligence.

The debate on the factors leading to hominin brain expansion is still ongoing and the lack of sufficient fossil evidence means we might never be certain about what actually triggered encephalization. However, there are things we know for sure: our human brains can collect, process and store unimaginable amount of information; they can find innovative solutions to problems; they can create abstract ideas. They made us the dominant species. However, there is a price to pay: our brains have enormous energy requirements, and – due to the large size – childbirth is painful and difficult for human mothers. There is no doubt, though, that it is only a small price for all the possibilities our brains give us.

WHAT FORCED US TO WALK UPRIGHT?

There are numerous theories trying to explain hominin bipedalism; however, none of them is entirely satisfactory, as each of them leaves many unanswered questions. In today’s post I will discuss the most important hypotheses of the hominin bipedalism evolution.

The earliest theory of bipedalism, suggested by Darwin in 1871 and widely supported until the 1960s, linked walking upright to the use of tools; free hands were supposed to facilitate making weapons for defence and hunting. However, fossil and archaeological records show that there is at least a 1.5 million year gap between the development of bipedalism and the earliest stone artefacts which date back to about 2.6 Ma (Harcourt-Smith: 2007). Even if we assume that the first tools were made of wood, and so weren’t preserved, the timing difference makes this theory rather unlikely.

More recent theories tend to take into account the relationship between the emergence of bipedalism and the climatic shift towards cooler and drier conditions during the Plio-Pleistocene. Initially, according to the Savannah theory, scientists thought that walking upright was an adaptation to shrinking forests and spreading grasslands. Upright posture would help to see over tall grass, reduce skin’s exposure to the sun, allow more efficient body cooling and facilitate carrying resources across open spaces.

However, as new paleontological evidence appeared, bipedalism started to be seen as one of the adaptations to the extreme climatic variability (not the encroaching savannah) in East Africa during the Plio-Pleistocene. This might be why the fossils of our ancestors show signs of both walking upright and climbing trees – such flexibility could have been crucial to succeeding in diverse habitats. This adaptation made it easier to gather food from the trees and the ground; it also facilitated carrying what was gathered over long distances.

Decreasing the dependence on trees could have favoured the development of bipedalism: some researchers believe that such a locomotion mechanism is very energy efficient on the ground.  A study published by Sockol et al. in 2007 showed that human walking needs 75% less energy than both quadrupedal and bipedal walking in chimpanzees. Then again, some other studies (like this one conducted by Halsey and White in 2012) suggest that, compared to other types of mammalian locomotion, bipedalism is similar in terms of its energy efficiency.

There is one more interesting hypothesis I want to talk about, put forward by Lovejoy in the 1980s. He argued that upright walking in humans was linked to monogamy. According to Lovejoy and his supporters, increased bipedalism facilitated carrying food to desired locations. As our ancestors became monogamous, females would choose a partner with the ability to provide plenty of food to her and their offspring. Consequently, upright-walking males would have been more likely to reproduce, passing bipedalism onto the following generations. However, this theory is quite controversial as it is unclear whether the early bipedal hominins were indeed monogamous.

It is evident that when it comes to human bipedalism, there is still a lot to be understood. Although the last 50 years were exceptionally abundant in fossil discoveries, the evidence is far from complete. There is still no clear answer or agreement on how hominins became bipedal; however, there is no doubt that the development of bipedalism was crucial to human evolution. It was fundamental to establishing our flexibility and resilience – the features that helped us become the dominant species.      

BECOMING HUMAN: BIPEDALITY

Walking upright seems so obvious and natural to us humans. In fact, the ability to walk on two legs is the trait that separated the first hominids from other four-legged apes, making bipedality one of the most fundamental human characteristics. Since the 1960s the fossil record has significantly increased, however scientists still cannot agree on why and how this radical change came about.

So let me start with what we can be the most certain about: the timeline of bipedality development. Here are some examples of the ancient roots of upright walking:

• The shape of the thigh bones of Orrorin tugenensis who lived 6 Ma suggest Orrorin was bipedal.
• The reconstruciton of the skeleton of Ardipithecus ramidus from 4.4 Ma shows extensive evidence for bipedality too.
• Lucy, the 3.2-million-year-old Australopithecus afarensis fossil, leaves no doubt for the species’ upright walking.

However, while showing signs of bipedality, all of the above species retained some features (such as long arms, short legs and long fingers and toes) indicating that they spent a considerable amount of time on trees. Homo erectus, ‘the upright man’ who appeared 1.89 Ma, was the first fully terrestrial hominid with an anatomy very similar to ours (Wayman: 2012).

What can we conclude from the current evidence then? Firstly, it suggests that the transition between quadrupedalism and bipedalism was gradual. Although first signs of walking upright appeared 6-7 Ma, it wasn’t until around 3 Ma when the early hominins became nearly as efficient at bipedal locomotion as us. They developed foot and pelvic bones which provided more support and stability for bipedal movement. The figure below represents the comparison of foot and pelvic bones between chimpanzees, Australopithecus africanus and Homo sapiens (Arsuaga and Martinez: 2006).  

 

Source: http://anthro.palomar.edu/hominid/australo_2.htm

The second conclusion we can draw is that, again, the past might have been much more complicated than we thought. We know that the hominin family tree used to be very diverse and it’s likely there was a high degree of locomotor diversity within early hominins too (Harcourt-Smith and Aiello: 2004). Different patterns of locomotion represented adaptations to a variety of habitats – which fits nicely with what I said about climate change and habitat fragmentation in my previous posts. In the next post I will explore different theories on the development of bipedality in hominins, as well as the possible relationships between walking upright and the changing climate.