The origin of bipedalism. A review

Becoming bipedal was a vital step in the path to humanity. Walking on two legs freed the hands of our ancestors enabling the crafting of tools, allowing a meat focused diet and ultimately leading to a bigger brain - the characteristic that has allowed us to become the dominant species we are today. We are the only mammal adapted to a life of walking upright; it is true that some Great apes do take to two legs but their bodies are not built for sustained periods of walking and they quickly revert to their more energy efficient movements.  

The transition to become a habitual 'upright ape' has not been an easy one; a variety of skeletal and muscular modifications were necessary to make bipedalism worthwhile, leading to back problems, hernias and extremely difficult child birth. So what factors would have made it necessary to leave a quadrupedal lifestyle which suited our ancestors for millennia?

The oldest and most well known theory to answer this question is the 'savanna hypothesis', suggested by Raymond Dart (1925). His idea being that bipedalism evolved in an open grassland setting, an environment created from receding rainforests.  It was here that ‘mammalian competition’ and lack of water provided the selection pressures required for an ape that relied on fast legs and a large brain to survive. At the time the hypothesis was proposed the earliest bipedal hominid – the Taung child was dated at 2.5 million years old (mya) (Dart 1925), ice core samples support a reduction in rainforest. They reveal that the climate at this time was indeed becoming cooler and dryer – a repercussion from the closing of the Panama seaway.

A classic image of early man in a savanna setting

A supporting hypothesis was put forward by Wheeler (1991), he proposed that walking upright would have allowed our ancestors to keep cool on the savanna due to a reduced surface area being exposed to direct sunshine and increased wind speeds away from the ground. These hypotheses seem simple and logical, however the discovery of the 4.4mya bipedal hominid –Ardipithecus ramidus pushes the origin of bipedalism back to a time when Africa was covered in thick rainforest; a finding which prompted scientists to seek new explanations.

A paper by Kevin Hunt (1996) gives a good alternative theory. It suggests that our ancestors developed their bipedalism whilst still in the trees; the idea was developed from an observational study of our closest living relatives – the Chimpanzee. These Great Apes use facultative standing to reach food items, holding onto overhead branches for support; Hunt argues that our early ancestors would have used a bipedal posture to attain food and this would have developed into efficient locomotion at a later point.

Modern Orangutans also use facultative standing to reach food items   

Unlike the Savannah hypothesis the postural feeding theory fits in with the rainforest environment that is predicted to have been present at the dawn of bipedalism, and is therefore in my opinion much more likely to be true. Other theories have also been put forward including bipedalism adapting to make sex organs more visible or to allow males to provide for their mates. The argument over which factor is the most likely is still ongoing, and this debate will be continued as new research emerges.

New research into this topic have put forward some new ideas, a 2011 paper by David Carrier proposes that sexual selection may have played a role. This is the selection of traits based on female preference rather than traits which will enhance survival, a bipedal stance would have enabled males to fight one another more effectively to gain mating rights to a female. Only males winning these fights would pass on their genes and therefore this gait would have been selected for.

Finally, a 2010 review of all bipedal hypotheses, concludes that no one hypothesis can explain our adaption towards a bipedal lifestyle, the issue is far more complex than that, an opinion I tend to agree with.

References

Dart, R.A., 1925. Australopithecus africanus: The man ape of South Africa. Nature, 115, pp.195–199.

Hunt, K.D., 1996. An ecological model for the evolution of bipedalism. South African Journal of Science, 92(2), pp.77-90.

Wheeler, P.E., 1991.The thermoregulatory advantages of hominid bipedalism in open equatorial environments: the contribution of increased convective heat loss and cutaneous evaporative cooling. Journal of Human Evolution, 21(2), pp.107-115.

Video time!

If you have a spare few hours here and there the series 'Walking with Cavemen' gives an entertaining narrative of human evolution. Although it glosses over some of the major debates in Palaeoanthropology it brings the world of these species to life, and Robert Winston's facial expressions are pretty hilarious too!

As the video isn't from youtube it won't let me embed them, but follow the links below for the episodes:

Episode 1 - Australopithecus afarensis

Episode 2 - Homo habilis/ Paranthropus boisei

Episode 3 - Homo erectus/ Homo ergaster

Episode 4 - Homo neanderthalis

The Genus part 2

At a similar time to Homo ergaster appearing in the fossil record, a  species called Homo erectus existed in Asia. Many people in the scientific community believe that these two species are interlinked and that H. erectus is just an Asian version of H. ergaster which migrated from Africa to colonise other climes. An alternative view is that H. erectus evolved completely separately in Asia and then migrated to Africa at a later date. An analysis will be conducted on this subject in a separate post so we can look at it in a bit more detail.

For now we'll push on with the biographies of the most influential species from our past.

H. erectus were a species of Homo who travelled a lot (assuming they evolved in Africa). In the 1.5 million years (around about) that they existed they managed to reach countries as far flung as China, India and Europe, surviving as hunter gatherers. Their brains were the largest to date at about 2/3 the size of ours, they were using this extra brain power for innovations such as ocean crossing by raft and controlling fire (although this is still a source of contention).

A trailer for the film 'Homo erectus'. Don't let the title fool you, this is a truly terrible film!

 

  Another species which has been found outside of Africa is that of Homo neanderthalansis; you may know them as neanderthals. In popular media this group has been described as the quintessential 'caveman' complete with bone club and a tiny intellect, however what you may not know is that they had brains actually larger than our own. Their morphology (structure) was more robust and muscly than ours and was evidently geared towards a life in cooler climes - a reflection of their commonly ice age habitat. Genetically neanderthals were very similar to us, so much so that there is debate about whether they should actually be a sub species of sapien instead of a seperate species all together. This would result in a reclassification of their name to Homo sapien neanderthalansis. They would have inhabited Europe 230,000 years ago until their extinction as recently as 28,000 years ago.  

Now, finally, we have reached the conclusion of our Homo safari, with a description of our own species Homo sapien. The earliest remains currently place the dawn of our species at 195,000 years ago, not too long after neanderthals; this was marked by a brain size equal to modern man and small teeth adapted for a varied diet.We quickly dispersed from Africa, travelling to Asia Europe and later the Americas and Australia, by 12,000 years ago we inhabited all continents except Antartica.

 

From left. H.erectus, H. neanderthalansis and H. Sapien.

The Genus part 1

Our species Homo sapien was not the only upright ape to develop in Africa, so far around 13 different species belonging to the genus Homo have been discovered (although this number is still debated). The characteristics which link them to one another are complex and would take up more space than this one post, I will therefore be mainly mentioning what is thought of as the key developments: an increasing brain size and a reduction in size of teeth. Bipedalism, which many people suggest was the biggest development in making us who we are today, was actually adopted by an even earlier ancestor 4 mya (million years ago) - Australopithicus.

Australopithecines were a genus of ‘upright apes’ and would have had a brain size similar to that of the Chimpanzee and teeth which were still much larger than our more recent ancestors due to their mainly vegetarian diet. They were living in eastern Africa until around 1.8 mya.

Next on the scene were Homo habilis 2.4 mya, this species is widely agreed to be the first true Homo and is notable for its increased brain size and a regular use of stone tools, there is also evidence of an increasing meat orientated diet, a change which would have helped boost brain size.

Homo ergaster was another important species to have developed, they were one of our direct ancestors and lived in southern and eastern Africa from 1.8 until 1.3 mya. They show modifications in gait which suggested a more upright posture and the development of running. This change would have allowed them to become more efficient hunters, extra resources which in my opinion could have boosted population numbers from an increase in numbers of offspring able to be supported. It has been put forward that H. ergaster may have migrated from Africa 1.75 million years ago, a topic hotly contested which will be discussed more fully in a later post.

From left to right - an Australopithecine, H. Habilis and H. ergaster

A Cladogram of the species mentioned:

http://throughevolution.files.wordpress.com/2011/09/hominina-cladogram31.jpg

For a more succinct view of the sequence of events check out this time line.

Next time will be the conclusion of 'The Genus'.

Beginnings

Hey Everyone,

As I'm sure you are aware, humans are everywhere! There is just no escaping them, we have spread to colonise almost every habitat on Earth. We have adapted the environment to suit our own purposes and increased in number to become the dominant life form on Earth. All of this from a small group of individuals which dispersed from Africa  around 1.7 million years ago. This blog is going to look at how the genus Homo accomplished this impressive feat. It will also look at the role climate has had in helping and hindering this journey and shaping the different ethnic groups we see today.

So buckle up and prepare yourself for an epic journey back to an age of Mammoths, Saber-toothed cats and giant Ground Sloths. We will visit ice ages and Sahara grasslands, examining the climatic shifts along the way. In this blog I hope to not just examine scientific papers but also bring you relevant videos and pictures which I have found interesting.

A Woolly Mammoth - painted by the people who hunted them

Hope you enjoy it!