Tetrapod expert gives a public lecture in Grahamstown

If we trace our ancestory back through time we rapidly reach our four-legged ancestors (or tetrapods). And if we follow that line far enough, say to 360 million years ago (in the Devonian Period), we meet ancestors who were also the ancestors of elephants, kangaroos, canaries snakes and all other land living creatures with backbones – some of whom have later lost their legs!  

 

Ahlberg came to South Africa at the invitation of local palaeontologist Dr Rob Gess to examine important new unpublished clues from shale deposits found at Waterloo Farm next to the N2 highway. 

He gave an informative lecture on the origins of Devonian tetrapods about 400 million years ago. The lecture entitled Devonian tetrapods, when, where and how was well received as it was crafted to be accessible to those who know little about palaeontology, yet interesting to the advanced enthusiast as well. 

Devonian tetrapods were strange four-legged animals that over millions of years made the transition from living in water to living on land. The word tetrapod means ‘four feet’ – so why do birds only have two feet and snakes have none? The answer is that the ancestors of both had four limbs, but in the case of birds, two of them evolved into wings while in the case of the snakes, all four limbs were lost in the process of evolution. 

Ahlberg explained to the audience how a group of large lobe-finned fish gradually transformed into a peculiar animal that looked something like a salamander and maybe behaved a little like a mudskipper. This, he said, was one of the most pivotal moments in vertebrate evolution, and ecological history – as the impact of their descendants on land ecosystems has been enormous.

He reviewed traditional thinking on the evolution of tetrapods and noted that developments in the last twenty years, including discovery of well-preserved intermediate forms,  have pulled the rug out from under dated text books - that often described fish dragging themselves onto land. Rather, tetrapods developed gradually within the aquatic realm, probably exploiting the shallows around lakes and lagoons.

Until very recently a series of forms from the Late Devonian of Europe, Greenland and North America (that formed a tropical landmass at the time) appeared to provide a neat sequence of forms, that illustrated a greatly improved textbook account of the emergence of legged vertebrates. Intermediate forms (or Élpistostegids’) had basic legs that still ended in fin webs rather than toes, lost their back fins and could move their heads like tetrapods. They had eyes on the top of their heads like alligators. Conveniently fossils of these creatures seemed to fall in age between those of conventionally fishy lobe-finned-fish, and the earliest tetrapods.

But as is often the case this neat new linear model has in turn been shaken up by new discoveries.
In the last five years, tetrapod track-ways from as early as the mid-Devonian have been identified in various parts of the world, indicating that known intermediate-type forms were living side by side with tetrapods.  In addition intermediate forms showing contradictory combinations of ádvanced’ and ‘primitive’ features have shown that the evolution of tetrapods is a lot more complicated, or ‘branchier’ than was earlier believed. 

Ahlberg said it was not always a straightforward exercise to tell which tetrapod or elpistostegid developed earlier or which came later because some species were advanced in one particular aspect while another species might be more advanced in completely different trait. It appears that the transition from fish to tetrapods was not an entirely linear progression. 

He said that as scientists now have a clearer picture of the morphological changes in the transition period, more questions remain to be answered. He also suggested that more intriguing creatures are still to be found that might help to answer these questions. 

Dr Gess has over the years discovered several remarkable fossils in the 360-million-year old black shale deposits he excavated from Waterloo Farm just outside Grahamstown. The fossils include, seaweeds and plants, a lamprey, a number of other fishes including a school of baby coelacanths and the pincer of an ancient scorpion. Per Ahlberg indicated that this site was unique for its age, a ‘lagerstatte’ in which the remains of an entire ecosystem are preserved.

Unlikely that Homo naledi lived alongside Homo sapiens

Prof Lee Berger’s announcement on Tuesday (9 May 2017) that the bones of Homo naledi have been reliably dated to between 236,000 and 335,000 years old is big news. It is also big news that his collaborators have found another chamber within the Rising Star cave system near Johannesburg containing yet more H. naledi  bones, including the almost complete skeleton of an adult male.

Narrowing down dates for the hominin fossils is remarkable because many palaeontologists, including Berger, had believed that based on their appearance, the bones were at least two million years old. If these estimates had proven to be accurate, it would have placed H. naledi  at the base of the Homo genus as one of the oldest species in our family tree.

The dating issue became controversial because the first bones were found in 2013 and presented to the public in 2015 without any evidence for their age. Some professionals insinuated that the absence of a reliable age for the bones showed that Berger and his team had rushed their investigations.

After a long wait, Prof Paul Dirks and his team published a paper this month in the online journal eLife showing that the concentration of radioactive elements and the damage they caused in three fossilized teeth mean that the fossils are most likely between 236,000 and 335,000 years old.

To ensure that the results were accurate, the paper confirms that “the most crucial tests were carried out at independent laboratories around the world, and the scientists conducted the tests without knowing the results of the other laboratories”.

The fact that the Rising Star fossils are much younger than had been expected does not necessarily mean that H. naledi  should be removed from the base of our family tree. It is possible that about 2.5 million years ago, there was a fork in the original family of H. naledi  where one part evolved into H. erectus and eventually H. sapiens, while the other side lingered on unchanged as H. naledi  for another two million years or so. 

While there is no evidence to support this possibility, it cannot be ruled out. It is quite probable that H. naledi  will turn out to be an evolutionary dead-end – an interesting one – but nevertheless quite dead.

However, what is likely to be the most controversial aspect of Berger’s announcement is his claim that “H. naledi  possibly lived at the same time, and in the same place, as modern humans”. This is the angle that led in most mainstream publications in South Africa.

The Port Elizabeth Herald declared unequivocally that “Homo Naledi lived with man” even though not a shred of evidence was presented to support this dramatic claim. The newspaper said, “About 300 000 years ago‚ South Africans shared their land with another human-like species: Homo Naledi”.

This statement created erroneous impressions that first of all, there were South Africans; and secondly that these South Africans lived in harmony with relatives of another species. We can almost imagine them breaking bread together and perhaps sharing a fraternal beer in the spirit of ubuntu.

News 24 online news site said, “scientists and researchers can now reveal that it is highly likely that the species lived alongside Homo sapiens” suggesting that they hung out together.

Even Wits Vice Chancellor Adam Habib got caught up in the political feel-good spin conjured up by Berger and according to the Herald said “The true significance of this research shows we belong to a common humanity. That is a fundamental thing.

“In a world of division … when people are looking at their ethnicity to define who gets what and in a world where we are becoming so intolerant of each other‚ this research shows we have very common roots. We represent a common humanity.”

Actually it doesn’t.

The real problem with portraying humans and H. naledi  as best buds is that there is no evidence to show that they were alive at the same time. If we take the youngest date for the existence of H. naledi  as being 236,000 years ago and the oldest date for anatomically modern humans as being 200,000 years ago – there is quite a large gap of around 36,000 years.

It is true that someone might find evidence to support the notion that H. naledi  is  younger or H. sapiens is older, but currently this possibility is not sustained. Things might change, but in terms of current research, Berger is stretching the boundaries of what might, conceivably be possible into the realm of what could be good publicity for his book - which coincidentally was published on the same day as his dramatic announcements.