EARLY INVADERS | Invading Ireland:

The Valentia Island Vertebrate and the Move on to Land

With last month's post being concerned with the evolution of the curious style of upright walking in our own line over the last six or seven million years, it seems a logical step this month to stride even deeper into the past and reveal the origins of walking in the vertebrate line as a whole and their move on to land. This is a fascinating story that has become increasingly interesting over recent decades, with new discoveries being made all over the world, one of the most important of which, as it fortunately turns out, comes from Ireland.

We can begin our story, then, in Valentia island, which lies just off the Iveragh Peninsula of Co. Kerry in Ireland's southwest. Up until quite recently, this island was best known for having been the location linking Europe to North America via the first commercially viable transatlantic telegraph cable, which began operating in 1866. The line operated for a hundred years before being decommissioned, but in 1993 a discovery was made that thrust Valentia back into the spotlight once more.

This discovery dated back almost 400 million years to a time before the Atlantic Ocean had opened, and Ireland and Europe were truly connected to North America as part of a massive continent called Laurussia, which also included Greenland and Scandinavia, as well as much of western Russia, hence the name.

Ireland lay well south of the equator at this time, sitting on the eastern fringes of a vast desert, which covered a great expanse of this gigantic continent, and Valentia Island was joined to the mainland as part of a great low-lying coastal plain occupying much of what is now Ireland's southernmost province, Munster.

In this unrecognisable Ireland, life was only to be found around the edges of lakes and ponds and the borders of some mighty rivers carving their way southwards from mountains in the north on their way to an ancient southern sea. However, one inhabitant of this ancient land left evidence of an everyday event which, nevertheless, signifies one of the most important milestones in the history of life on Earth.

One day, around 385 million years ago, this metre-long animal slowly plodded across a layer of silt which had recently been deposited by one of Ireland’s ancient rivers, which occasionally flooded the southern coastal plains. By chance, these footprints were buried under a fresh deluge of sediment and over time hardened like concrete, only to be exposed again millions of years later in the location they now sit, high on the sea cliffs of Valentia Island.

These footprints are of enormous significance in the quest to understand our own origins, as they belong to a time when the first vertebrates were making the transition to land.

That the Valentia Island vertebrate walked does not necessarily mean that it was a land animal, though. The evolution of fish into the first four legged vertebrates, or ‘tetrapods’ ('tetrapod' meaning ‘four feet’ in Latin), is no longer thought to have been spurred by the move on to land.

Walking may actually have first evolved as an adaptation to living in the shallow waters of a lagoon, as it uses less energy than swimming and may also have been useful when hunting. It was only later that fins originally modified to walk along the lagoon floor also came to be used, perhaps, to drag these fish out of the water twice daily to feast on the dead and dying animals which had been stranded by the tide. The brief but regular terrestrial visits by these transitional ‘fishapods’ eventually promoted the evolution of true legs, feet, and toes, giving the world its first tetrapods.

But even these first tetrapods were not really land animals. The transition of vertebrates to land was an extremely gradual process, taking place over many millions of years along the margins of land and sea.

The earliest evidence for tetrapods on Earth – a series of footprints discovered recently in Poland and dating to around 395 million years ago – illustrates this point quite clearly. Although they show the clear imprints of feet and toes, it is perhaps telling that they seem to have been made by animals floating in water. These creatures were probably unable to fully support their bodies in the same way on land, especially given that some of them could reach two-and-a-half metres, or more, in length.

Even 20 or 30 million years after these footprints were made, tetrapods still seem to have been quite aquatically adapted and it is only around 350 million years ago that we have firm evidence of a tetrapod that seems to have been truly able to walk on land. As it happens, this ancient, metre-long tetrapod, called Pederpes, lived very close to Ireland, in western Scotland.

But where does all this leave our Valentia Island vertebrate?

There has been much debate over the true nature of this animal and whether it made its footprints while walking on land or submerged in water, but recent discoveries like the footprints in Poland perhaps shed some light on the possibilities.

These older footprints show that it is at least possible that this animal was a tetrapod, although the lack of digit imprints in the Valentia Island tracks means that they could also have been made by a walking fish. Whether or not this animal was submerged when it made these footprints it is harder to tell, and while some later fossils which remain quite aquatic in nature seem to suggest that an animal millions of years older was unlikely to have been able to walk on land, we must be aware that some of these are thought to have belonged to a divergent lineage from the one which would lead to fully terrestrial forms and may have actually been evolving back into a more aquatic lifestyle, so earlier tetrapods could conceivably have been more capable on land.

In any case, whether walking fish, aquatic or semi-terrestrial tetrapod, the Valentia Island animal retains its importance as one of the earliest forms on the road to the vertebrate conquest of the land. From these pioneers, a new world was created as their descendants spread across the lands of the Earth and diverged greatly in form, some even going one step further and invading the air.

Just looking around at the most recent descendants inhabiting Ireland today, it is very easy to see just how different they are. From birds sailing in the sky to lizards scurrying under rocks, proud-antlered deer in forests to slick-furred otters in streams, house mice hiding in shadows to the humans whose homes they inhabit, vast ages of evolutionary separation from one another is written in every aspect of their form and behaviour. And yet, we can ultimately trace all of their evolutionary origins back to the same beginning, to a time around 400 million years ago when fish-like forms first emerged from the sea...