Saturday, 28 September 2013

Face To Face

Our ability to spot a human face often leads us to anthropomorphise
objects we see around us. One instance of this is the so-called 'Face on Mars'
Humans are social creatures. It was our ability to pick out an individual from a crowd which led to our dominance of the planet as we forged relationships with each other, built family groups, tribes and eventually empires.

We are programmed to spot a human face, whatever the circumstance. People have even reported seeing them carved into the surface of Mars or the Moon. Similar instances of this natural mimicry can be found across the planet. Sometimes they come from the animal kingdom.

The plan of our skulls and the placement of our features are simple adaptations of the same basic plan. Evolution cannot easily create new structures. Instead it changes what already exists. Enlarge the jaws and teeth, slope the face, stretch out the brain case and make everything smaller overall and you have the skull of a chimpanzee.

While these skulls may look very different, they
all share the same basic design
The skulls of jawed vertebrates all possess the same basic layout. The size, shape and function of the parts is what makes jawed vertebrate skulls, properly called gnathostomes, appear different. Look at a horse and you will see it has two eyes, a jaw, teeth and nose. The same goes for a bird, a shark, an elephant or any gnathostome.

Even in creatures which have lost the use of sight, such as cave-dwelling amphibians like the axolotl, the skull still has two eye sockets.  

Today, the group accounts for 99% of all living vertebrates. Some jawless vertebrates have survived, such as lampreys or hagfish, but they are primitive in comparison. As a result, there have been extensive efforts to find out how jaws evolved. It was originally thought that a group of now extinct, armored fish known as the placoderms first developed jaws. However, as they died out without leaving any evolutionary descendants, palaeontologists hypothesised that 'modern' jaws must have evolved separately in the placoderms' closest ancestors the cartilaginous fish, chondrichthyes.

The 419 million year old cranium of the placoderm
Entelognathus primordialis
In turn they passed the structure on to the more advanced bony fish, osteichthyes, the closest ancestors of land-living vertebrates.

A fossil discovery from China is set to revolutionise our understanding of where our faces came from and how the vertebrates as a group evolved. Earlier this year, a placoderm fossil was unearthed from the Kuanti Formation near Quijing. Dated to around 419 million years old the specimen, named Entelognathus primordialis, was remarkably well preserved, more so than most placoderm fossils.

As a result researchers, led by Min Zhu from the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, were able to study its morphology. They found something rather surprising; that Entelognathus possessed a complex arrangement of bones (premaxilla and maxilla on its upper jaw, mandibles on its lower jaw and cheekbones) previously only thought to exist in the skulls of cartilaginous and bony fish and land-dwelling vertebrates. Yet it is older than all three groups. 

From this, the research team has suggested that this 419 million year old creature may represent the common ancestor of all jawed vertebrates At the very least its immediate antecedants are the common ancestors, as some studies have suggested that placoderms and the other groups of fish split away from each other 421 million years ago, just 2 million years before Entelognathus evolved. If they are right, then this creature is indeed amongst the earliest animals on the planet to have a face.

An artist's impression of Entelognathus, the oldest known face on the planet
Indeed it is a simpler solution to the problem of evolving jaws. In the previous model, vertebrates would have had to have developed the structure twice, an occurrence known as convergent evolution. While we know that such a process is responsible for the evolution of many body parts, most commonly the eye, it is generally an unfavourable solution. Instead, most palaeontologists prefer to construct evolutionary stories in a fashion similar to Occam's Razor, using the simplest methods and pathways possible.

Alongside the eye, the backbone and the brain, the evolution of the jaw should be considered one of the most important events in the history of the vertebrates, as gnathostomes account for 99% of the group. The jaw, alongside specialised teeth, is what allowed vertebrates to occupy the sheer diversity of ecological niches they inhabit today, giving them the ability to out-compete virtually every other organism they encountered. Our faces indeed carry a deep evolutionary legacy.