One of the earliest human ancestors - that lived 400 million years ago - was a 'fish' with no jaw that used spines on its head to shift position in the water, study claims.
Researchers from the University of Bristol used computer simulations to create 'avatars' of our ancient relatives to explore how they moved through the water.
Named Cephalaspis, the creature was about a foot long - roughly the size of a modern trout - and heavily armoured with a thick bone from snout to tail.
The study suggests the jawless aquatic creatures started to diversify body and head shapes to suit different different environments much earlier than first thought.
The team behind the study hope to use the same computer modelling technique to understand the whole diversity of 'early' vertebrates and their evolution.
Impression of Cephalaspis, a typical osteostracan, swimming over the substrate. The creature was about a foot long - roughly the size of a modern trout - and heavily armoured with a thick bone from snout to tail
Cephalaspis was a member of the osteostracans species - regarded as 'lazy lumps' that sat at the bottom of rivers and seas feeding on algae and worms in the mud.
A remarkable feature of the fish is that it lacked a chin, fins and legs but had striking spikes on its head - the Bristol team say it used this to shift position in the water.
OSTEOSTRACANS: ANCIENT BOTTOM FEEDING 'LAZY LUMPS'
Osteostracans - Latin for 'bony shells' - is a long extinct jawless fish species.
The armoured 'fish' lived in what is now North America, Europe and Russia.
They featured a shield of bone stretching from the head and down through the tail
They spanned from the Middle Silurian to the Late Devonian eras.
There were among the most advanced of all jawless fish from the period.
This is in part due to a more complex brain that held memories.
They featured a shield of bone stretching from the head and down through the tail.
The creatures had sensors in the skin that could detect the smallest vibrations in the water.
Recent studies have revealed they may have been good swimmers and widely adapted to suit different aquatic environments.
This is in contrast to earlier studies that suggested they were 'lazy lumps' that stayed on the bottom of a river or sea.
The new study revealed that the species were much more adept in the water than believed - with some able to move on the bed and others freely in the open water.
Computer simulations showed the strange spikes and spines that ornamented their skulls were actually hydrodynamic adaptations.
They enabled the animals to passively generate lift from water currents flowing over their body - giving them movement through the water.
The varying head shapes of different species allowed them to thrive in a variety of positions within the water - from high to low.
The groundbreaking discovery shows they were already ecologically diverse - long before the arrival of jawed vertebrates.
Co-author Dr Humberto Ferron, of the University of Bristol, said the evolution of jaws and fins have classically been seen as the key inventions that allowed vertebrates to diversify their lifestyles.
It was previously thought that the jawed vertebrates out competed the 'lumpy' species and eventually went on to dominate and move to the land.
'In this context, jawless ancestors, characterised by the presence of heavy rigid headshields, were assumed to be cumbersome fish-like creatures, living on the bottom of rivers and seas, with poor manoeuvrability,' said Ferron.
Dr Ferron and colleagues used state-of-the-art computational engineering techniques to mimic their behaviour based on fossilised remains.
Co author Dr Imran Rahman, of Oxford University's Museum of Natural History, said using computational fluid dynamics allows them to study the swimming performance of long-extinct vertebrates with no modern equivalent species.
'Our main finding supports that the adaptions that allowed the diversification of vertebrates within aquatic environments occurred before than jaws evolved,' Dr Ferron told MailOnline.
'This challenges the widely accepted paradigm that the first jawless fish-like vertebrates were basically sessile organisms living on the substrate.'
It also shows that 'the emergence of jaws and paired fins triggered the diversification of lifestyles in vertebrates.'
The ancient fish-like creatures weren't just adaptable - they were clever too - with brains capable of holding memories of breeding grounds.
They would have also been prey for large arthropods - monster sea scorpions and giant crabs and used sensors on their skin as an early warning system.
A digital recreation of a ostracoderm. The varying head shapes of different species allowed them to thrive in a variety of positions within the water - from high to low
Researchers used fossils of ostracoderms to produce their computer simulationa nd determine how they were able to swim
When breeding season came, Cephalaspis congregated in the one place they could escape the scorpions - inland fresh water - with their convoy plowing upriver.
They returned to the spawning grounds where they hatched, thanks to one of the first complex brains that allowed them to hold the location in memory.
It was much more developed than their rivals who had no memory at all, and allowed Cephalaspis to process information - and escape.
The team hope to use the same computer simulation techniques developed for this study on other ancient vertebrates.
'We plan to apply this technique to more groups spanning the whole diversity of "early" vertebrates to further check some other classical hypotheses on the early evolution of this group,' Dr Ferron told MailOnline.
'Is really exciting methodology as it allows for very broad analyses and it is still quite novel approach in palaeontology.'
LIFE ON EARTH MAY HAVE STARTED THANKS TO A MODIFIED VERSION OF MODERN-DAY RNA
Life on Earth may have started thanks to a modified version of modern-day DNA's sister molecule, scientists believe.
DNA is the backbone of life and almost all of our planet depends on it but, on primordial Earth, a primitive version of its lesser-known sister - RNA - was the focal point for evolution, experts say.
RNA is structurally similar to DNA, except one of the four fundamental pieces, thymine, is substituted for uracil.
This changes the shape and structure of the molecule and researchers have long believed this chemical was vital to the development of Earth's first lifeforms.
Scientists claim that a chemical called inosine may have been present in place of guanine, allowing for life to develop.
This slight change to the bases, known as a nucleotides, may provide the first known proof of the 'RNA World Hypothesis' - a theory which claims RNA was integral to primitive lifeforms - they say.