In 2004, three palaeontologists found a fossilised fish that might have been the first back-boned animal to venture onto the land, a venture with fundamental consequences for us all. Called Tiktaalik, this fish had a neck, shoulders, and fins with primitive elbows and wrists. In January this year, the same team described Tiktaalik’s hind fins and pelvic girdle, another significant finding for the biological community. I spoke with Professor Neil Shubin, one of the discoverers of Tiktaalik, and Professor Colin Groves, an anthropologist and co-discoverer of an ancient human ancestor: Homo ergaster. The discovery of these fossils prompts us to contemplate our closer human ancestors, our more distant ancestors, and explore the deeper significance of Tiktaalik and its recently described hind fins.
A reconstruction of Tiktaalik roseae. Photo: Alina Zienowicz CC
How far back in your family tree can you imagine? Mother, grandmother, great-grandmother? It is easy to picture your most recent ancestors. They are real people whose voices, appearances, and even smells you recognise, but what about your great-great-grandfather? How familiar are you with him? As you follow this unbroken chain of people back into the past, they get more and more abstract, and ever more difficult to imagine. What happens to their height as you move back in time – did they live during times of famine? What colour eyes did they have? Who was the first individual to join your lineage that would pass on the genes to make your eye colour? By using our imagination, we can only get so far in picturing our greatest grandparents. Fortunately, imagination is just one tool we can use to chip away at this question; another is palaeontology.
Homo ergaster or ‘the working man’, a human ancestor that lived in Africa one and a half million years ago. Photo: Staffan Vilcans CC BY-SA 2.0
When animals with bony skeletons pass away, a record of their presence is sometimes left in the Earth. We can unearth the history of our own species and determine our relationships with all other species through the science of palaeontology.
Palaeontology is the methodical search for fossils, from which details can be gleaned about the creature that left them behind and the environment they lived in. To find fossils we must dig deep. Generally, the deeper you dig, the further through time you travel.
There are many places in the crust of the modern-day Earth that reveal different points in time. We can trace the history of our own human skeletons backwards through time and see the transformation of our primate bodies. Digging back through the timeline of our own ancestors we find Homo sapiens, Homo heidelbergensis, Homo erectus, andHomo ergaster.
Looking at these individuals, we can see the steady decrease in the size of our brain case and the augmentation of our hyoid bone required for speech. A grandparent such as Homo ergaster, whom we all share, would have been communicating with one another, albeit with more simplistic sounds. Further back through time, we find Australopithecus, an ancestor who walked on two legs, but who had a protruding jaw and large canine teeth. Tracing our grandparents back through ancient history, we see their anatomies metamorphosing in a smooth transition from one form to another. We can draw a line between two fossils and define them as unique species, but between the vast sequence of individual mothers and daughters that connected these species, no line could be drawn.
But this is just the sequence of our own human evolution. We are simply mammals. What about other mammals, or birds, or reptiles, or amphibian evolution? The bird outside the window, she has great-grandparents too. Consider the unbroken chains of ancestry for all of these animals. Are they connected, and when was their most recent common ancestor?
At some point in Earth’s history lived the most recent common grandparent to llamas, finches, turtles, axolotls, and you. We may never know if a particular fossil is that exact ancestor, but in 2004 three palaeontologists found a fish that ticked all the boxes.
Beginning in 1999, vertebrate palaeontologists Ted Daeschler, Farish Jenkins, and Neil Shubin began searching the Canadian Arctic for a fish fossil that possessed the characteristics of a tetrapod ancestor.
Tetrapods are animals with a backbone and four limbs such as ourselves. The team was studying rocks that dated back to the Late Devonian, a period when there were no tetrapods and the only back-boned animals were fish. At this stage of Earth’s history, large animals were confined solely to water; life had not yet stumbled onto the requirements for movement on land.
Professor Neil Shubin, fish palaeontologist, paints a picture of the Late Devonian: “If you were to take a time machine and visit the world about 375 million years ago, what you have are some of the earliest forests…primitive trees, shrubs, you have all kinds of invertebrates living on land. You have no creatures with bony skeletons, none whatsoever.”
The creature Professor Shubin and his colleagues were searching for would come to be the first exception to that rule.
Finally, after five years of searching, the team found what they were looking for: Tiktaalikroseae. This fossilised ancient fish possessed the first instances of features necessary for life on land; Tiktaalik, it appears, may have paved the way for us all. Tiktaalik was a large, predatory fish, which grew up to three metres long.
It had many of the characteristics you might imagine of an estuarine fish living in shallow water bodies. Tiktaalik was scaly like a fish, had a flat skull with eyes on top and strong jaws lined with sharp teeth. Unlike a fish however, Tiktaalik’s front fins projected from shoulders attached to its spine, whereas other contemporaneous fish had fins that projected from their skulls. The fins of Tiktaalik were bony and muscular with primordial elbow and wrist joints, allowing for sophisticated arm movements.
Further, Tiktaalik had a rib-cage robust enough to support its own weight outside the buoyant medium of water. Tiktaalik was well and truly a hybrid with characteristics of both a fish and a tetrapod. The skeleton of Tiktaalik was incomplete however; the back half of the animal was yet to be described.
A giant mudskipper, Periopthalmodon schlosseri, a modern day example of an amphibious fish capable of locomotion on land. Tiktaalik was once thought to haul itself across mudflats primarily with its fore fins, similar to mudskippers. Photo: Ben Lee CC BY-NC-ND 2.0
In January 2014, ten years after the initial finding, the scientists that discovered Tiktaalikpublished a paper titled “Pelvic girdle and fin of Tiktaalik roseae” in the journal Proceedings of the National Academy of Sciences. Their findings came as a surprise to themselves and the larger biological community.
Before January it was assumed thatTiktaalik moved around on land by pulling itself forward with its strong fore fins and dragging behind its otherwise fish–like hind fins similar to the locomotion of modern–day amphibious fish like mudskippers. “We were kind of surprised at the whole thing,” said Professor Shubin, lead author of the paper describing the finding. “The closest relatives to Tiktaalik on the fish side have really tiny pelvic girdles relative to their shoulders—really tiny—and relatively small, weak fins.
When I first saw the pelvis ofTiktaalik it was way bigger than I thought it would be, relative to the shoulder. It’s as big as the shoulder, in fact, if not bigger in some ways. [The pelvic fins are] capable of supporting it by walking in the water, as a paddle. You can think of this as an amphibious kind of animal, and think about the pelvic fin as an organ that would allow it to meet a lot of functions in the water.”
This body plan that Tiktaalik had struck upon, of four limbs of equal size and strength, was the start of a great thing – the start of the tetrapods. To discuss the significance ofTiktaalik’s large pelvis and hind fins for the kingdom of life, I spoke with Professor Colin Groves, a palaeontologist and anthropologist at the Australian National University.
I asked Colin which animals would come to inherit this four-limbed system. “All of us,” he replied. “Right from Tiktaalik onwards, through the amphibians, the reptiles, to the birds and mammals. We’ve all inherited that. We’ve obviously elaborated the hind limbs, and also the pelvis, which we [now] know was…reasonably developed in Tiktaalik. This is a legacy that we owe essentially to Tiktaalik and its contemporaries.”
Tiktaalik, or a fish like it, is our ancestor – but not just ours, that of all tetrapods: kangaroos, woolly mammoths, and even snakes and velociraptors. Shared grandparents like Tiktaalik can serve as a reminder we are animals, primates, and all other animals are our cousins to varying degrees.
There are great benefits for our society in recognising humans as another species in the tree of life. “Evolutionary discoveries and evolutionary models put humanity in perspective,” said Professor Groves.
At the time of this interview, Prime Minister Tony Abbott was attempting to begin a ‘renaissance of logging’ in the world heritage old-growth forests of Tasmania. In a speech to promote the plan, the Prime Minister adapted passages from the Bible proclaiming that the environment was given to Man, and we would be unintelligent not to make the most of it.
I asked Professor Groves what benefits our society might gain by acknowledging that we too are animals. “At the moment, the benefit is that politicians, economists, big business, people like that, tend to regard humanity as sort of being apart [from the animal kingdom],” Groves said.
“They can do what they like, as if we are not subject to the same environmental constraints as ‘the animals’. We can treat ‘the animals’ as we like and that’s a widespread thing. The environment is changing because of what humanity has done, and it will affect us just like it will other species.
The idea that we can shelter ourselves from environmental change, for example, is a bit of nonsense that is very well entrenched in the minds of many politicians and in big business in particular.”
Our imaginations can only take us so far in discovering who we came from. Our sciences, like palaeontology, take us beyond abstract imaginings and bring us face-to-face with our distant ancestors. Tiktaalik roseae is a special ancestor. It represents the historical journey when life moved from familiar waters to the alien land.
Tiktaalik possessed some of the first mutations that shaped our human bodies: a neck, shoulders, two arms, hips, and two legs. Not only did Tiktaalik shape our form, but it also shapes our identity – we are primates in the tree of life. We are descended from distant ancestors that crawled from the ocean to the land, then to the trees and down again.
We were not given the forests, we came from the forests, and we owe it to the Earth to preserve and cherish the birthplaces of ourselves and all of our cousins.