Study shows studies of the origin of vertebrates – including human – were based on incorrect assumptions since the early 1900s
A new study published in Nature is prompting the rewrite of all biology textbooks relating to the origin and evolution of vertebrates – including the modern human.
The study, by Professor Rob Gess from the Albany Museum in Makhanda (Grahamstown) shows that the foundation on which most of the research done on the origin of vertebrates (any animal with a backbone such as goldfish, lizards, crows and people) has been based on incorrect assumptions.
For the past couple of centuries, scientists have been studying modern lamprey fish in an attempt to establish the origins and evolution of vertebrates, as it was believed that the modern lamprey can be seen as a “time capsule” that can provide clues as to how, when and why vertebrates evolved.
Lampreys are eel-like in shape and feed by latching onto other fish with a round sucker that surrounds their mouth, securing their grip with circles of teeth on the sucker and then drinking their victim’s blood after rasping a hole with special teeth on their tongue. Adult lampreys are therefore, extremely successful, having arisen before the first four-legged-animals moved onto land and survived, with little change, ever since.
“Lampreys and modern hagfish are the only jawless fish alive that branched off from the family tree of vertebrates before they got jaws,” says Gess. “This made them very interesting for researchers as this suggested that modern lampreys were swimming time capsules that could give unique insights into the biology and genome (DNA) of a truly ancient lineage.”
While this, in many ways this are true, Gess’ research shows that the above assumption only relates to adult lampreys.
By studying lamprey fossils from the 360 million-year-old Waterloo Farm black shales near Makhanda (Grahamstown) in South Africa, Gess found that the ancient juvenile lampreys hatched from their eggs with a completely different set of features than modern lampreys do.
“Modern lampreys hatch as blind, filter-feeding, worm-like larvae (ammocoetes) that burrow in stream beds and filter water for minute food particles, before slowly transforming into free-swimming, eyed, actively feeding adults,” says Gess.
This strange life history of the lamprey was thought to echo transformations some 500 million years ago, which gave rise to all fish lineages, including the one that ultimately led to vertebrates – and humans.
“For this reason, the last invertebrate ancestor of vertebrates is often portrayed as ammocoete-like, and the earliest vertebrate as being lamprey-like. But for this to be a reasonable model, both ammocoetes and lampreys would need to hark back to the dawn of our (vertebrate) history.”
However, Gess’ new fossil discoveries contradict the conventional wisdom that our long chain of ancestors ever included a lamprey-like fish. Painstaking excavation of shale samples from Waterloo Farm have revealed a growth series of Priscomyzon - an exquisitely preserved lamprey fossil from Waterloo Farm which Gess described in Nature in 2006 –illustrating its development from hatchling to adult. Remarkably, the smallest preserved individuals, barely 15mm in length, still carried a yolk sac, signalling that these had only just hatched before entering the fossil record.
Gess’ crucial discovery showed that the hatchlings represented in the fossils were already sighted with large eyes and armed with a toothed sucker, much like the blood-sucking adult phase of modern lampreys and completely unlike their modern larval counterparts.
“This drastically different structure of ancient lamprey infants provides evidence that modern lamprey larvae are not evolutionary relics,” says Gess.
Gess believes the modern filter-feeding phase is a more recent innovation that allowed lampreys to populate and thrive in rivers and lakes. Less complete (previously unpublished) partial growth series of three types of slightly younger lampreys from North America support this finding.
“Distant human ancestry seemingly did not include a lamprey-larva-like stage. Lampreys now appear to be a highly evolved side branch, which shared a common ancestor with us – probably a jawless fish enclosed in bony armour.”
“The Albany Museum is really proud of Dr Rob Gess’ latest spectacular discovery, which is one of a number published in the world’s most prestigious journals. Rob’s lifetime efforts in conserving the Waterloo Farm site, collecting, curating and researching the fossils that have been excavated over many years, is an accomplishment for which our community can be really proud. One of our treasures is the Waterloo Farm fossil collection, curated at the historic Old Priest House in Beaufort Street. This collection is of great international significance and attracts researchers from all over the world. The fact that Rob Gess’ research is on a local fossil site in Makhanda is a massive bonus for our little town, and for our Museum. The Waterloo Farm collection only exists because of Rob’s tireless efforts, at times undertaken at his own expense. His foresight, vision and effort have brought immense credit to South African palaeontology and highlighted our country’s rich fossil record,” concludes Mr Manzi Vabaza, Manager of the Albany Museum