“IT STARTS with a question,” says Ted Daeschler. “What are the watershed moments in the history of life?” He is talking about evolution’s most innovative twists – how birds took flight, when humans split from chimps, and how jaws originated, for example. To discover how these changes unfolded, we need what palaeontologists call transitional fossils – better known as missing links.
For Daeschler, who is based at the Academy of Natural Sciences in Philadelphia, Pennsylvania, the big question was: how did terrestrial animals evolve from fish? The key step, so we think, was when the lobe-shaped fins of some bony fish evolved into limbs around 375 million years ago. But there was no fossil evidence for this. So Daeschler teamed up with Neil Shubin from the University of Chicago and together they scoured geological maps in search of surface rocks of the correct age. Their spotlight fell on Ellesmere Island in the high Canadian Arctic and, in the fourth summer of digging, they finally uncovered their quarry: a fish with four limbs. They called it Tiktaalik.
When the discovery was announced in 2006, it made headlines around the world. People were captivated not only by the fossil, but also by how it had been found. In Darwin’s day, naturalists collected anything of interest and many discoveries were made by chance. Daeschler’s search was more systematic. “Tiktaalik,” he says, “was a great example of a prediction that you could make and go out and validate” – by discovering the right fossil.
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Today’s missing-link hunters are increasingly taking such a tack, making predictions and then using a variety of advanced tools, ranging from gene sequencing to modern imaging techniques, to find the fossil evidence. It’s an approach that is allowing palaeontologists to make big strides in understanding evolution’s innovations all along life’s timeline.
Life began well over 3 billion years ago; the ratio of carbon isotopes in 3.8-billion-year-old rocks from Greenland bears its fingerprint. But finding fossils from so far back is problematic. The very first link in the chain of evolution might not have fossilised and, even if it did, it could be unrecognisably different from anything alive today. Microscopic structures in Australian rocks 3.4 billion years old may be the oldest physical traces of life found to date. Not everyone is convinced.
However, thanks to genetic analysis, we are gaining important insights into another early transition. Rocks dating from about 2.4 billion years ago contain oxides of iron which signify the appearance of photosynthesising organisms that released oxygen. It has long been assumed that these organisms – known as cyanobacteria – originated in the sea. But when Carrine Blank of the University of Montana in Missoula sequenced the genomes of modern cyanobacteria to compile a family tree, she found strong evidence that all the ancestral species lived in fresh water (). That suggests photosynthesis began in fresh water, probably between 3 and 2.5 billion years ago. Guided by this insight, Blank’s colleagues are focusing on deposits from ancient lakes and streams in their search for fossil cyanobacteria (see timeline).
The first photosynthesisers were single cells lacking a nucleus. Then came organisms with cell nuclei, the oldest known fossil of which dates from around 1.8 billion years ago. Evolution’s next major trick was the emergence of multicellular life – a key missing link. Studies that use rates of genetic change to extrapolate backwards from today’s creatures indicate that the first animals appeared between 1 billion and 600 million years ago. Fossils of that age are very rare, though, and even where they exist, it is difficult to distinguish between simple multicellular life and colonies of single-celled organisms.
The Ediacarans – mostly spongy, rippled mat-like creatures rooted to the seabed – emerged around 585 million years ago. They vanished abruptly 542 million years ago in a burst of evolutionary change known as the Cambrian explosion. We can trace the origins of most major animal groups alive today to this period, which saw the evolution of the first animals with hard body parts, and the first predators. This transition changed the living world so profoundly that the evolutionary connections between Cambrian fauna and the Ediacarans remains an enigma.
Recent detective work has, however, shed light on a key innovation that happened in the seas around 100 million years later. “One of the biggest unsolved mysteries in evolution is the gap between jawless and jawed vertebrates,” says John Long of the Natural History Museum of Los Angeles County. The evolution of jaws was vital to the success of species from sharks and Tyrannosaurus rex to humans. “It required a rearrangement of the whole cranial anatomy,” Long says. Now we know when and where that anatomical rearrangement got started. In 2011, Zhikun Gai at the Institute for Vertebrate Paleontology and Paleoanthropology in Beijing, China, announced that he had found a crucial intermediate form (). He made the discovery using synchrotron X-ray microscopy, which gives 3D images at submicrometre resolution without damaging samples.
The specimen Gai scanned, that of a jawless fish called a galeaspid, had already been studied under optical microscopes, but the X-rays revealed something new hidden within the rock. Rather than the single nostril found in other jawless fish, Gai found a pair of nostrils, one on each side of the skull. In more advanced fish, the space between paired nostrils allows the embryonic cells that form jaws to migrate to the correct position. So although galeaspids remained jawless, by evolving paired nostrils they removed a barrier to jaw development. Gai now plans to work forward from jawless fish to look for other signs of emerging jaws.
Some 70 million years after this particular galeaspid swam the oceans, fish made another evolutionary leap when they took to life on land. Tiktaalik and its kin were the first fish with limbs strong enough to walk on the shore, but they were not proper residents. True amphibians evolved tens of millions of years later. Exactly how and when is hidden by a break in the fossil record called Romer’s gap, named after the palaeontologist who first noticed it, Alfred Romer. Why fossil evidence from this period, from 360 to 345 million years ago, is so sparse is still debated, but we know that it followed a mass extinction at the end of the Devonian, the Hangenberg event, that wiped out many primitive fishes. Lobe-finned fish and the paddle-limbed descendant of Tiktaalik, Acanthostega, vanished. So did archaic armoured fish called placoderms, including the fearsome 10-metre-long Dunkleosteus. After the gap, more modern ray-finned fish and sharks dominated the seas, and amphibians with salamander-like legs roamed the land.
Recently, several were discovered. To fill in the gap properly, palaeontologists are planning an audacious project: drilling a borehole 500 metres deep through rocks that include buried parts of these fossil beds. The idea is to obtain a core spanning the mysterious 15 million years. The , named after the drill site near the Tweed river on the border of Scotland and England, will cost almost ÂŁ400,000 and be funded by the UK Natural Environment Research Council. Such costly drilling is rare in palaeontology, but the researchers believe it will pay dividends. Microfossils from the core will serve as an index that can be used to gauge the age of nearby fossil sites and how they relate to each other, potentially demystifying the rise of the amphibians.
Elsewhere, palaeontologists are making progress understanding the most devastating extinction of them all. Up to 96 per cent of all marine species and 70 per cent of terrestrial vertebrates died out in the end-Permian extinction, 252 million years ago. The next 20 million years saw the rise of the archosaurs, the “ruling reptiles” that gave rise to all birds, crocodiles, dinosaurs and pterosaurs. The first archosaur fossils appear at the start of the Triassic. Soon afterwards, the group seems to have split into lines leading to dinosaurs and crocodiles, and this is where things have been a little hazy. Ctenosauriscus, dating from around 247 million years ago, was only identified as an early relative of crocodiles in 2011 (). Footprints of a dinosaur or a dinosauromorph, its close cousin, found in Poland in 2010 are even older. But until very recently, the oldest known dinosaur fossil was just 230 million years old.
Last year, a specimen some 10 to 15 million years older than that turned up. Nyasasaurus came as a surprise, not least because it had been sitting in the collection of the Natural History Museum in London for decades. Both it and Ctenosauriscus were identified as missing links thanks to a growing database of early archosaur specimens. With software to compare physical traits across many species, we can make connections not immediately obvious from skeletons. For example, Ctenosauriscus has a sail on its back, but computer analysis revealed many less obvious features that put it among the crocodilians. Nyasasaurus was spotted as a potential missing link by Sterling Nesbitt of the Field Museum of Natural History in Chicago, who does fieldwork at the Manda beds in Tanzania where it was unearthed. Sure enough, his computer-aided analysis revealed it to be an early dinosaur or close relative ().
Nesbitt is also using a more commonplace tool that could have been a boon to past fossil hunters. Before returning to the Manda beds last year, he and his colleagues consulted Google Earth. “It turns out we had been walking past a big outcrop, maybe 30 metres off the path, but we could not see it through the tall grass,” he says. When they got back to the site, they found that the outcrop was packed with fossils.
Despite these successes, one group of archosaur descendants remains elusive. Pterosaurs evolved in the early or mid Triassic, but they had hollow bones, and their ancestors were probably small and lightly built, so fossils are rare. “Nobody really knows where pterosaurs came from,” says Stephen Brusatte of the University of Edinburgh, UK. “We are missing a pterosaur archaeopteryx.” (See “Ancient poster child“.)
Like amphibians and archosaurs, modern mammals evolved following a mass extinction: the asteroid impact 66 million years ago thought to have killed off the dinosaurs. But the mammals that emerged in the subsequent 10 million years “are still an enigma”, says Thomas Williamson of the New Mexico Museum of Natural History and Science in Albuquerque. Recent digs have turned up some links to modern groups, including ankle bones that show the oldest known primate, Purgatorius, was climbing trees 65 million years ago in what is now Montana. But other connections remain tenuous, and the oldest fossils of groups such as bats and whales do not appear until much later. Williamson and Brusatte are using techniques similar to those employed in the hunt for early archosaurs to try to understand how mammals appeared and diversified.
Meanwhile, the hunt continues at a handful of fossil beds where early mammals are preserved with soft tissue intact. In 2009, Darwinius, a 47-million-year-old fossil, was found in the Messel pit site near Darmstadt, Germany. Nicknamed Ida, it made a big splash as a possible missing link in the lineage leading to humans. We now know it was on a side branch that went extinct, but Darwinius nonetheless gives important insight into the anatomy of early primates.
As genetics has helped disentangle the origins of the earliest life forms, so it has cast light on the mysteries of our own evolution. For example, we now know that the split between the human and chimp line happened far earlier than was once thought. Using mutation rates as a molecular clock reveals that our common ancestor lived between 7 and 13 million years ago. (An earlier and less accurate clock had suggested it existed between 4 and 6 million years ago.) Besides pointing the fossil hunters to deposits of the right age, the discovery may lead to a reassessment of ape fossils that had been deemed too ancient to be relevant.
The biggest achievement of genomics in palaeontology, however, has been to give meaning to fossil scraps. By themselves, the isolated partial finger bone, tooth and toe bone found in the Denisova cave in Siberia told us little more than that hominins had lived there about 40,000 years ago. But sequencing the DNA revealed that these unremarkable bones came from a previously unrecognised human lineage that shared ice-age Eurasia with Neanderthals and modern humans. The discovery goes far beyond finding a lost cousin: about 5 per cent of Denisovan genes survive today in the people of Papua New Guinea. Similarly, analysis of Neanderthal DNA suggests that many of us have a dash of Neanderthal ancestry.
Of course, some discoveries are still made using old-fashioned exploration. A decade ago, the world was stunned by Homo floresiensis, the “hobbit”. Its skeletons were found on the Indonesian island of Flores, where it lived until at least 17,000 years ago. Then last year came the revelation that another group of primitive humans, the Red Deer Cave people, appear to have lived in what is now China until even more recently. If researchers could find a way to extract DNA from their remains, our understanding of human evolution would be hugely enhanced.
We are better equipped than ever to solve evolution’s outstanding mysteries. High-tech tools aside, today’s palaeontologists also have one key advantage over their predecessors: knowledge accumulated over generations. In the field, what might once have been discarded now has a much better chance of being recognised. Nesbitt, for instance, identified an unassuming 10-centimetre-long bone as the femur of an archosaur, leading to the discovery of an entire skeleton of Xilousuchus, an early crocodilian.
“Almost every day there’s something new and exciting, a new fossil find or publication,” says Brusatte. “It is a golden age.”
See a timeline of life’s missing links
Ancient poster child
Archaeopteryx remains the iconic transitional fossil, a small predatory dinosaur caught in the act of evolving into a bird about 150 million years ago. Its discovery in 1861, just after publication of Darwin’s On the Origin of Species, could not have been better timed. It would be another 130 years, however, before palaeontologists found a treasure trove of feathered dinosaurs and birds with similarly exquisite preservation. These fossils, at the Jehol formation in China, have raised a host of new questions about bird evolution, particularly how small dinosaurs evolved wings and flight feathers. However, they are some 20 million years younger than Archaeopteryx.
Despite its fame, Archaeopteryx is rather late in the day as far as the shift from reptiles to birds is concerned. Researchers would really like to find transitional specimens that predate it, and were excited by the discovery a few years ago of older fossil beds beneath Jehol. Unfortunately the Daohugou formation has not lived up to expectations so far: it has yielded only about 20 species and no birds, says David Hone of Queen Mary, University of London. The absence is a puzzle, but it’s early days. Besides, the beds have yielded another key transitional fossil. At 160 million years old, Juramaia sinensis is the oldest known placental mammal, pushing the split with marsupials back 35 million years ().
This article appeared in print under the headline “Find the gaps”
