Life in transition: A tale of two fossils

Two spectacular fossil finds announced in the last year show us evolution in action.

 
  Museum fossil collections are a wonderful opportunity to see species change over millions of years.   Brendan Lynch/Flickr  (CC BY-SA 3.0)

Museum fossil collections are a wonderful opportunity to see species change over millions of years. Brendan Lynch/Flickr (CC BY-SA 3.0)

The fossil record is one of the great archives of life’s history on earth. The world’s museums are filled with extraordinary specimens that cover and catalogue the astonishing diversity of life, from the present to the almost unimaginably distant past. Although modern molecular genetics provides equally irrefutable evidence for evolution, there is something tangible about fossils that is unique ‒ the ability to see and appreciate these extraordinary animals up close.

Yet despite the wonderful exhibits that adorn halls on every continent, popular misconceptions remain. Some refuse to accept the reality of a vastly ancient Earth; others are unconvinced that humans are merely a tiny and recent twig on an enormously packed bush.

Two wonderful fossil discoveries from the past year may help erode these misconceptions: the hominid Homo naledi, discovered in a cave in South Africa, and the “four-footed snake” Tetrapodophis amplectus, originally from Brazil but rediscovered in a German museum.

It is no surprise that gaps still exist in the fossil record. The first limitation imposed by nature is statistical ‒ the chance of any animal becoming a fossil is actually pretty slim. It needs to die in the right location (preferably near a stream bed), avoid being devoured by hungry predators, and be covered by mud and sediments fairly quickly after expiring. Even after decomposition, the bones aren’t necessarily safe: in the past and present, there has never been a shortage of predators capable of munching through bone.

Once the remains fossilise, they require just the right geological movement to expose the rock strata to the surface, but not for so long that wind and erosion permanently erase what remnants there are. When you consider that palaeontologists have only been wandering around for the last 200 years (out of approximately 400 million years for the vertebrate fossil record), the amount of material we have preserved and displayed is remarkable.

  The discovery of  Archaeopteryx  provided early support for Darwin’s theory of evolution. Pictured here is the Berlin specimen, discovered in 1874/75.   H. Raab/Wikimedia Commons  (CC BY-SA 3.0)

The discovery of Archaeopteryx provided early support for Darwin’s theory of evolution. Pictured here is the Berlin specimen, discovered in 1874/75. H. Raab/Wikimedia Commons (CC BY-SA 3.0)

A particularly fascinating group of fossils are ‘transitional fossils’, which seem to sit across two different groups of animals, possessing characteristics of both and providing insight into how one species evolved into another. Given the slow, gradual process of evolutionary change, this is perfectly predictable, but it wasn’t always understood or accepted. Charles Darwin himself in The Origin of Species acknowledged the lack of transitional forms as a clear problem to his theory, in a chapter admirably titled “Difficulties on Theory”.

Remarkably, only two years after the publication of Darwin's book, the first specimen of Archaeopteryx was discovered in Germany. This animal possessed very clear reptilian features, such as teeth and a bony tail, alongside bird-like feathers, hollow bones and fused wrist bones. In the 155 years since, more and more transitional forms have turned up. There are now transitional fish, amphibians, reptiles, dinosaurs, birds, mammals, frogs, turtles, snakes, lizards, elephants, horses, cats, bears, seals, rhinos and, most spectacularly, whales (Melbourne Museum now displays a wonderful cast of Ambulocetus natans, the “walking, swimming whale”). The absence of transitional forms in 1859 has become a gigantic surplus in 2016.

Recently adding to this surplus is Tetrapodophis amplectus, an organism whose name literally translates to “four-footed snake.” It was discovered in Germany’s Museum Solnhofen by paleontologist David Martill from the University of Portsmouth, who happened to walk past the previously unidentified fossil and noticed what others had missed. T. amplectus supports previous assertions that snakes evolved from a lizard-like ancestor: the fossil has distinctly snake-like features, but there is also no mistaking its four protruding legs, including digits. The specimen dates from the Cretaceous period, around 120 million years ago, in what is now Brazil. 

  The fossil of  Tetrapodophis amplectus , showing its full snake-like skeleton (top) and protruding limbs (bottom).  David Martill/Helmet Tischlinger/Nicholas Longrich (used with permission)

The fossil of Tetrapodophis amplectus, showing its full snake-like skeleton (top) and protruding limbs (bottom). David Martill/Helmet Tischlinger/Nicholas Longrich (used with permission)

Previous fossil snakes had been found with hind limbs, such as Eupodophis descouensi from 92 million years ago. But T. amplectus takes us one step further back; it shows beyond doubt that snakes evolved from ancestral lizard-like forms. Yet another gap in the fossil record has now been filled.

Another area of the fossil record where amazing finds continue to be unearthed is the hominids. For almost a century after Origin of Species, the physical record of hominid ancestors was sketchy and uncertain. There were rare but spectacular early finds, with Rene Dubois discovering ‘Java Man’ from the East Indies in 1894 (since reclassified as Homo erectus), and Raymond Dart finding Australopithecus africanus in 1924 in South Africa. Debate had ensued for decades as to whether Man originated in Asia or elsewhere, with Darwin himself proposing Africa as the likely origin in his 1872 The Descent of Man. As with many other things, in time Darwin was proven to be completely correct.

The popular ‘March of Progress’ illustration paints evolution as a linear process moving inexorably from knuckle-walking hominids to Homo sapiens in all its glory. In reality, the hominid evolutionary story, like those of other species, is a densely packed bush. There is one hominid species remaining today, but in the past there have been many species and multiple genera. Until humans supplanted the Neanderthals around 40,000 years ago and became the dominant species, many forms coexisted over long periods of time. In the last 50 years, a range of extraordinary specimens have shown the ancestors of H. sapiens to be far ranging and extremely diverse. Firmly held preconceptions about Man’s origins have been re-evaluated and re-examined time and time again.

The most recent addition to this lineage was discovered in South Africa in 2013, in an astonishing trove that housed some 1550 individual fossils from as many as 15 individuals. Announced late last year, Homo naledi sits somewhere in our hominid past, but, as is frequently the case, its true place in the evolutionary tree (or more accurately, the evolutionary bush) is unclear.

  A sample of the 1550  H. naledi  skeletal pieces recovered at a single site in South Africa.   Berger et al. (2015)  (CC BY 4.0)

A sample of the 1550 H. naledi skeletal pieces recovered at a single site in South Africa. Berger et al. (2015) (CC BY 4.0)

So far it appears that H. naledi possesses traits most like Australopithecus, but with some exceptions. The fingers, shoulders and ribcage are most like australopiths, whereas the wrist, palm, legs, feet and ankles resemble those from the Homo genus.  However, the one clearly conspicuous feature of H. sapiens – a big brain – is definitely not present. The braincase of H. naledi is only 560 cubic centimetres (cc), compared to an average of 900cc for H. erectus, and a whopping 1350cc for H. sapiens. Much more information is needed to determine exactly where H. naledi sits in relation to us.

Establishing exactly when H. naledi lived has also been problematic. A constraint on the discovery of any fossil is that only igneous rocks can be properly dated using radiometric dating techniques. Fossils are contained within sedimentary rock, but we can still infer their age by dating appropriate bookends of the deposit, such as volcanic ash layers. Time will tell whether this process can be performed on these spectacular finds.

Despite the importance of the H. naledi deposit, it is not without some controversy. Respected paleoanthropologist Tim White from the University of Berkeley has raised questions about the speed of publication, with the discovery published in the open access journal eLife instead of more known publications such as Nature and Science. By comparison, White’s discovery of the 4.4 million year old hominid Ardipithecus ramidus – discovered in 1994 – took a full 15 years of careful preparation and analysis before being formally announced in 2009. White also points out that physical traits claimed to be unique to H. naledi are present in H. erectus.

So far, our inability to accurately date the fossil site makes it difficult to establish any definitive timeline. So is H. naledi a new species, or are the fossils variations within a population of H. erectus? The list of hominid precursors grows longer and longer, but their contemporaries along the chimpanzee line have proved more difficult to come by. Are these likely to be found, or do the mostly forested habitats of chimps rule this out?

These two fossil discoveries from 2015 each enhance and underscore basic truths about the history of life on earth. Human beings diverged from a common ancestor with chimpanzees and bonobos around 7 million years ago, and are the sole remaining bipedal primate from a long line of hominid predecessors. Snakes may slither along the ground, up in trees, or swim in water today, but they evolved from a lizard-like ancestor that had four legs. Transitional fossils continue to be discovered all over the world, so let's hope 2016 yields similar treasures.

Edited by Andrew Katsis and Ellie Michaelides