Human teeth 'first evolved 400 million years ago' in a bizarre armoured fish, according to new X-ray analysis of a fossil.
Palaeontologists have used the European Synchrotron Radiation Facility (ESRF), the strongest X-ray source in the world, to 'digitally dissect' the primitive jawed fish.
The teeth, belonging to the extinct 'acanthothoracid' fish, were discovered near Prague in the Czech Republic almost 100 years ago.
However, they are trapped in encasing rock and scientists have never been able to study them before.
But the non-destructive X-rays revealed hidden details on their size and structure, showing a striking resemblance to human teeth.
Humans and all 60,000 living species of jawed vertebrates – sharks, bony fish, amphibians, reptiles, birds and mammals – are descendants of this fish, researchers say.
'These findings change our whole understanding of the origin of teeth,' said study co-author Professor Per Ahlberg, a palaeontologist at Uppsala University in Sweden, who led the research.
'Their jawbones resemble those of bony fish and seem to be directly ancestral to our own.
'When you grin at the bathroom mirror in the morning, the teeth that grin back at you can trace their origins right back to the first jawed vertebrates.'
Teeth in current jawed vertebrates, including humans and sharks, reveal some consistent patterns.
For example, new teeth usually develop on the inner side of the old ones and then move outwards to replace them.
In humans, this pattern has been modified so that new teeth develop below the old ones, deep inside the jawbone.
There are, however, several differences between bony fish – and their descendants, including humans and other land animals – and sharks.
For example, sharks have no bones at all and have a skeleton made of cartilage, and their teeth simply sit in the skin.
In bony fish and land animals, the teeth are always attached to jaw bones.
Trying to understand how all these different species have similar teeth has long been a mystery and researchers had focused on fossils of a group of ancient fish that lived about 430 to 360 million years ago, called the arthrodires.
This extinct order of fish are called a stem species, a common ancestor to many different descendants. They were believed to be the first jawed vertebrate in which teeth were known to exist.
However, palaeontologists struggled to understand how they could have evolved into the teeth of modern vertebrates because the differences between bony fish and sharks are so vast.
The European team of researchers, which included employees of the Natural History Museum in London, therefore set out to determine whether arthrodires were our ancestors or merely a specialised offshoot off the lineage.
The problem with acanthothoracids is that their fossils are rare and always incomplete.
The very best acanthothoracid specimens come from the Prague Basin in the Czech Republic, from rocks that are just over 400 million years old, and were collected at the turn of the last century.
They have proved difficult to study by conventional techniques because the bones cannot be freed from the enclosing rock, and have therefore never been investigated in detail.
The team therefore have turned to the unique properties of the ESRF to visualise the internal structure of the fossils in 3D without damaging them.
At the ESRF, a 2,770-foot (844 metre) ring of electrons travelling at the speed of light emits high-powered X-ray beams that can be used to non-destructively scan matter, including fossils.
'The results were truly remarkable, including well-preserved dentitions that nobody expected to be there,' said lead study author Valéria Vaškaninová from Uppsala University.
Follow-up scans at higher resolution helped visualise the growth pattern and even the perfectly preserved cell spaces inside the teeth's dentine – the layer of material that lies underneath the enamel of the tooth.
Like arthrodires, the acanthothoracid dentitions are attached to bones.
Sharks, meanwhile, are specialised in having teeth that are only attached to the skin – in contrast to the common perception that sharks are primitive living vertebrates.
But in other ways, acanthothoracid dentitions are fundamentally different from those of arthrodires.
Like sharks, bony fish and land animals, acanthothoracids only added new teeth on the inner side of the old ones, while the oldest teeth were located right at the jaw margin.
In this respect, the acanthothoracid look remarkably modern.
'To our surprise, the teeth perfectly matched our expectations of a common ancestral dentition for cartilaginous and bony vertebrates,' said Vaškaninová.
The tooth-bearing bones also carry small non-biting dentine elements of the skin on their outer surfaces – a characteristic shared with primitive bony fish but not with arthrodires.
This is an important difference because it shows that acanthothoracid jaw bones were located right at the edge of the mouth, whereas arthrodire jaw bones lay further in.
The results have been published today in the journal Science.