Taung, the first Australopithecus discovered, had an brain cast

South Africa is home of the original “missing link”  human evolution. It’s where Raymond Dart found the first Australopithecus; confirming we evolved in Africa from an ape-like creature. Since then, dozens of key discoveries have come out of the “Cradle of Humankind“. 

However, it seems like each discovery is raising more questions than answers. Did Homo naledi bury their dead? Why did Au. sediba walk so weird? What did the human cow eat? Clearly, our picture is incomplete and there’s still a missing link or two to be found.

Now, scientists have found out why all these missing links are, well, missing! And whilst it’ll likely stay that way for the forseable future1.

The found link

We’ve found a lot in the more than 30 caves that make up the Cradle of Humankind. Excavations there have uncovered at least 4 human species spanning the past 3 million years, with more discoveries on the horizon. With all these finds, how can there still be a missing link?

To answer that we have to travel north to the East African Rift. This is a region that also contains dozens of hominin fossil sites from the past few million years. Like the Cradle, several new species were first found here, along with famous Lucy fossil. 

Olduvai Gorge in East Africa, showing many layers with datable deposits (dates in brackets in million of years)2.  

However, there is one crucial difference between the two areas. The Rift is geologically active, with volcanos launching stuff across the area. Scientists can easily date these materials and, by extension, the sites where this debris lands2

Meanwhile, the Cradle is more geologically lazy. The caves there were formed gradually by erosion, with no volcanos mixing in datable materials. Without a solid chronology, our understanding of the Cradle remains confusing and jumbled1

So Robyn Pickering worked with a group of researchers from around the world to try and fix this. They began examining the calcium formations (called flowstones) produced by water dripping into caves. Thanks to advances in Uranium-lead dating we can finally figure out how old these are1

Now, researchers have been doing this for the past few years; already dating a few fossils from the Cradle of Humankind. However, Pickering et al. took it to the next level by dating several sites in the area with this technique. This allowed them to produce a fairly thorough chronology for the whole area1

Dates for different sites obtained by Pickering et al.

The (wet) missing link

This thorough chronology would be a big deal all on its own. It provides vital context to the Cradle, helping us figure out what on earth is going on there. But as cool as that is, it wouldn’t make for a sufficiently clickbaity blog headline. 

The really interesting part (and cool title) comes from what isn’t there. There are huge gaps between the dates they obtained, which becomes more apparent when you plot it out as a histogram. Pickering et al. were able to pull out 6 phases where flowstones formed (marked in blue on the graph) with 7 gaps inbetween1

The number of flowstones from any given point (black bar) versus a climate proxy (blue bar)

Since flowstones are formed by water dripping into caves, this shows the climate of the region was cycling between wet (when flowstones formed) and dry (when none did). Human fossils only come from one of those periods, creating a missing link in the othersup>1. 

Why were there these missing links? Perhaps the climate change meant thatt the Cradle went uninhabited for huge swathes of time. Or maybe sites from that period simply weren’t preserved, being lost to the ravages of time1.

Regardless, it’s clear that there is a big chance for a missing link (or several) to slip through these gaps. The record of the period will be patchy and incomplete, and our understanding of evolution will remain incomplete.

And the idea of a real missing link makes for a much better blog title.

References

  1. Pickering, R., Herries, A.I., Woodhead, J.D., Hellstrom, J.C., Green, H.E., Paul, B., Ritzman, T., Strait, D.S., Schoville, B.J. and Hancox, P.J., 2018. U–Pb-dated flowstones restrict South African early hominin record to dry climate phases. Nature, p.1.
  2. Walter, R.C., Manega, P.C., Hay, R.L., Drake, R.E. and Curtis, G.H., 1991. Laser-fusion 40Ar/39Ar dating of bed I, Olduvai Gorge, Tanzania. Nature354(6349), p.145.

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5 Comments

Ashley Haworth-roberts · 13th December 2018 at 4:43 pm

https://www.nature.com/articles/s41586-018-0711-0
Very interesting. But the first law of young earth creationism is that if things like ‘missing links’ (or the Oort Cloud) are missing or unobserved then they simply don’t exist. And the second is that evidence eg fossil evidence that is being interpreted in an ‘unbiblical’ way that clashes with the book of Genesis is evidence that is simply being misinterpreted – according to an unbiblical and false worldview.
However, if uranium–lead analyses can be used to date layers of flowstone by linking these to very wet periods (and fossils found within caves to intervening drier periods) that may help with chronologies. I assume the thinking is that individuals of known or possibly unknown species that died in the very wet periods might be missing from the fossil record in the area of the ‘Cradle of Humanity’ (of course the creationists attribute the bulk of this planet’s fossil record to Noah’s Flood).
The Abstract of the paper you reference suggests that fossil preservation is restricted to ‘drier intervals’. However you appear to be suggesting (having I assume read the whole paper) that ”it seems that the Cradle went uninhabited for huge swathes of time” and that the authors ”speculate that they might be drier periods” (drier not wetter – when I was assuming the caves might have been partially flooded).

    davidbump · 13th December 2018 at 5:50 pm

    “Very interesting. But the first law of young earth creationism is that if things like ‘missing links’ (or the Oort Cloud) are missing or unobserved then they simply don’t exist. ”

    Not quite. It’s that the default assumption is that they don’t exist. Science is supposed to be a field based on scepticism that leads to observation and demonstration. Certain aspects of astronomy, geology, and evolutionary biology, however, are based on the assumption that certain things must have existed or happened, so any appearance otherwise must be misleading (e.g. Dawkins admission that living things appeared to be designed, BUT…) and anything that is missing must simply have been lost and not found (yet), and maybe never will be found, as this case seems to be suggesting.

    Evolutionary biology is an exceptional area of science, which began with Darwin, who never made any predictions that weren’t self-fulfilling unless he also proposed an escape hatch, or even suggested there was good reason not to expect to ever get positive results. And indeed, we still haven’t seen all the vast number of long lines of development leading up to the first appearance of all the major phyla in the Cambrian deposits, nor a viable transitional form from land or arboreal animals and either pterosaurs or bats.

    Evolutionists and creationists alike work within their own frameworks of belief which dictate certain things and disallow others, and both have things they must explain away or make excuses for. We actually have a number of things in common as opposed to certain other groups, such as the post-modernists.

    Adam Benton · 16th December 2018 at 1:45 pm

    You’re right, I made a typo. It’s actually drier periods when there were fossils, not the other way around. I’ve updated the post to clarify this.

davidbump · 13th December 2018 at 5:36 pm

“We’ve found a lot in the more than 30 caves that make up the Cradle of Humankind. Excavations there have uncovered at least 4 human species spanning the past 3 million years,”

Fascinating! I didn’t realize all those sites were caves! Which “4 human species” did you have in mind? Are you counting Australopithecines?

    Adam Benton · 23rd December 2018 at 10:33 am

    Many fossils from this period tend to come from caves, simply because they provide some protection from the elements making fossil preservation more likely. That’s what makes the rift valley unusual – despite containing another few dozen sites – since it’s all open air stuff, preserved by volcanoes rather than caves.

    Hominins unique to the Cradle – in roughly chronological order – are Australopithecus africanus, Paranthropus robustus, Australopithecus sediba, and Homo naledi. The Little Foot fossil, painstaikingly extracted over the past few decades, may also wind up being a new species. Homo erectus and Homo sapiens have also been found there, along with a few other places.

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