Sex. Doing the dirty. A bit of the old in and out. Making teenage boys giggle and girls blush since the beginning of time. Or was it? Has sex always been around? If not, who ever thought that an awkward fumble followed by a nine month wait was a good way of continuing the species? Perhaps sex hasn’t always been the taboo subject that it is now, and we know that there are other ways than doing it like they do on the Discovery Channel, but the origin of sexual reproduction is still far from being resolved.
Why, for example, is it so prevalent? Why would you go to the effort of finding a mate and performing some intricate act with them, if you can more efficiently clone yourself instead? Cloning takes just one to make one more, rather than sex, which needs a matching pair to reproduce. There must be a good reason to keep such an inefficient process going.
And indeed there is. When it comes down to it, sex is not about the bump and grind, or the fancy feathers and elaborate mating rituals. It’s all about what goes on in the cells. Meiosis is the first stage, with a ‘normal’ cell dividing twice to make gametes with half the right amount of DNA – in our case sperm and eggs. Two gametes, usually from different individuals, come together at fertilisation, making up the full complement of DNA, and then the fertilised egg, or zygote, starts dividing and keeps on dividing until it has the right number of cells to make an organism. The whole thing is a lengthy and complicated process that is seemingly designed to confuse biology students.
But there is meaning behind the madness. Sex is what allows organisms to mix up their genes – first during the random allocation of gene variants to gametes, and secondly during the random choice of fertilising pairs – and is the reason that no two sexually reproducing organisms look identical. One of the greatest misconceptions in evolution is that random mutation in DNA drives the variation exploited by natural selection, when in actual fact it is sex. Sex effectively shuffles the genetic material of a whole species every time an organism reproduces – a much more effective way of experimenting with variation than waiting a few generations for just one, potentially damaging mutation to one gene.
So, that’s why sex is a more attractive prospect than cloning yourself. But it doesn’t explain how, when, or where it got started. When was the first time the earth moved? Do we owe the discovery of sex to animals, or were they latecomers to the sexual arena, following in the footsteps of earlier, smaller, and simpler organisms in the deep past? Scientists have no firm answers yet, but there are a few clues.
When trying to work out when something evolved, scientists have two choices. They can look at living organisms and use the information contained in their DNA to work out when it first appeared. Or they can use the fossil record to trace a creature back in time to when it first appeared. Both methods are useful, but both are beset with problems. The creature in question may survived until today, or its fossil record may be incomplete and misleading. This is just the case with sex.
Sex is easy to spot and study in living organisms today. In addition to bizarre sexual practices, like the mid-act cannibalism practiced by many praying mantids, or the colour coded treasuries of Australian bowerbirds, there are some more clear morphological adaptations that can be spotted in living or dead creatures. First and most obvious, is sexual organs. Chances are, if you are a sexually reproducing animal, you are going to have some specialised organs to do so whether you’re a barnacle with the largest penis in the world or boast a four-headed member like the modest echidna.
Sexual dimorphism is an indirect consequence of a sexual lifestyle. Most notable amongst elaborately plumed birds, like peacocks or birds of paradise, sexual dimorphism is also extremely realised in terms of body size in certain species of fish. The male angler fish, arguably the smallest vertebrate in the world, is forty times smaller than the female.
But all these products of sex appeared at a late stage in the history of cellular sexual reproduction. If we want to find the origin, we need to look at modern examples of much more primitive creatures. Many eukaryotic organisms (those with a nucleus, as opposed to bacteria) consist of just a single cell, and still manage to reproduce sexually without all the adaptive trappings. And these single-celled creatures, called protists, evolved much earlier than animals. So does sex predate the animals? Possibly. Assuming that those sexually reproducing protists that are living today have been doing it ever since they evolved, then yes. But what evidence do we have to prove this is the case? Like the ancestors of humans haven’t always walked on two feet, perhaps today’s protists haven’t always been reproducing sexually. We need to turn to the fossil record.
Tracking sex though the fossil record is a tricky business, mostly because of the extreme bias in what can be preserved. The bulk of the fossil record is a record of hard parts and sex is generally concerned with the softer parts of anatomy (no jokes). Whether occurring in animals or in tiny protists, the adaptations and cellular products of the sexual process are not made of hard mineralised substances and so are easily decayed away after death. Sexual dimorphism too is difficult to recognise unequivocally, with different sized skeletons often interpreted as different species, or as younger forms, rather than different genders. So the fossil record of sex must be built on the rare examples where soft parts are preserved, or the indirect effects that sexual reproduction may have on a species.
Two big clues both come from around a time of major revolution in the biological world, the so-called Cambrian explosion of animal life, around 540 million years ago, when all the animal groups appeared in the fossil record for the very first time. This was not only a time of extraordinary fossil diversity, but also of exceptional fossil preservation. Microscopic algae and intricate macroscopic animals are preserved in the finest possible detail. Amongst these, just before the appearance of animals, scientists found tiny remains which looked like balls of cells, with different number of cells in each – one, two, four, eight, sixteen, and so on. In fact, these fossils looked just like embryos. The now infamous Doushantuo embryos, named after the formation in China from which they were first described, have been the subject of intense scientific debate for the last 15 years, with many scientists arguing that they are just giant bacteria. If they are embryos though, then they provide evidence that the sexual processes that must have formed them were well established before the majority of animals emerged.
A second piece of evidence, and one which supports the similar idea that sex got started before animals, is more indirect. Given that today, sex is responsible for much of the variation we see amongst eukaryotic organisms, it is reasonable to assume that when sex first got started, it would have been marked by a sharp increase in the variation of the creatures alive at the time, which would be recognised as a peak in species diversity. Looking for peaks in diversity is quite easy in the fossil record, and once compared with information from other time periods, there remains one gigantic spike – the Cambrian explosion itself. Could the invention of sex by eukaryotic protists have sparked the beginning of the animal kingdom as we know it? Quite possibly.
The evidence from the fossil record seems to point to an origin of sex just before the evolution of the animals, and as far as we can interpret it, the patterns of diversity of living creatures seems to point in that direction too. It may be that we will never be able to place an exact date on the origin. But it is clear that without sex, without that mucky, clumsy, long-winded process that is the bane and the joy of so many, we wouldn’t be here – not just those who are able to read this text, but every animal alive today.
This article was written for, and originally published in, Aberdeen University Science Magazine Issue 3
Last night it was my pleasure to attend the Oxford Alumni Society Professional Networking Event at the Oxford and Cambridge Club in London. Just a week on from my disastrous visit to the London Cabaret Awards, the evening couldn’t have been more sensible, smooth, or better executed. I didn’t manage to lose a single item of clothing, and I arrived at Pall Mall a very fashionable five minutes late.
The event melded the seemingly non-sequitur fields of volcanology and evolution with public policy, with speakers Professor David Pyle and Dr Matt Friedman sharing with us some of the new and most relevant findings of their work in the elegant and homely environs of the O&C Club.
Following a jolly hour of wine and high-class nibbles, newly reunited with leavers from my graduation year and my fourth year seminar group from that morning, we crowded into the teensy lecture room and gradually settled down to learn.
Professor Pyle, who taught me volcanology as an undergraduate, started us off with an endearing familiarity. He spoke of Oxford’s involvement with the recent ash disruption from the April 2010 Eyjafjallajokull eruption in Iceland, demonstrating with some strong images, just how light the ash fall was. He spoke of his primary research interest – the interaction between volcanoes and the glaciers that commonly form in their craters, in places like the Chilean Andes.
Particularly close to my heart was his work on the Greek island of Santorini, where I spent an extremely pleasant field trip in my final year of undergraduate, and which seemingly involved riding around in boats and admiring the view. Prof. Pyle and his geophysical colleagues at Oxford have been monitoring the dormant crater at the centre of the island since 2004, with a hope of detecting changes in the shape and seismic activity of the volcano that may signal an upcoming eruption.
And they may have spotted just that. Measurements of the tiny movements of the rocks since January 2010 have hinted at a bulging in the centre of the island complex, which is likely to be cause by a pulse of magma surging upwards to fill a magma chamber. Exciting stuff, and I am by no means enough of a volcanologist to know what this might mean in the future, but it is fascinating to be able to document the real-time activity of a classic Mediterranean volcano, and to potentially compare it with those historical eruptions responsible for the decimation of the Minoan civilisation, and the levelling of Pompeii. Happy happy memories of those halcyon undergraduate days when beer was cheap and the sun always shone…
Dr Matt Friedman, molester of fishes and general vertebrate whizz, followed and, coping well with the inevitable technological stall, treated us to some excellent pictures, movies and reconstructions of fossil fish. I never knew they could be so interesting!
As a paleaontologist, I have always turned my research attentions to the squishier, smaller and weirder parts of the early fossil record. But I learnt more from Matt in half an hour than I did for my entire undergraduate course. I learnt that of all the vertebrates, over half of them live in water. I learnt that you can douse your precious fish fossil in acid to make its bones stick out more to study them, and I learnt that some now extinct fish looked really, really stupid.
Matt has the rather dubious honour of being the flatfish fossil king, and has used the skeletons of some primitive groups to show how fish evolved from having one eye on each side of their face, to having both on the same side. It apparently involved an evolutionary stage where they looked sillier than usual:
Some of the really cool work that Matt is doing at the moment makes use of probably the biggest piece of scientific kit in the UK – the diamond light source synchrotron. The synchrotron accelerates particles around the huge doughnut-shaped building, generating x-rays that can be used like a super high-powered hospital CT-scanner to peer inside some exceptionally preserved fossils. Despite having only just started this, Matt and his coworkers have already got some exciting results, being able to reconstruct the delicate gill supports, and the nerves inside the skull of some early fishes.
Both speakers got plenty of incisive questions from the diverse audience, and as we hurried back to the wine and nibbles, I heard nothing but enthusiasm, for the lawyers and linguists, as well as the easily-pleased geologists.
Having been in the same department in Oxford for the last eight years, it is easy to feel staid, and tied down by the expectations and traditions of some of the older members of the faculty. It was truly refreshing to be a part of the younger, outward-looking and truly outreaching new generation of Oxford scientists. I came away glowing with pride and wine, and hoping that researchers and teachers like Prof. Pyle and Dr Friedman can help Oxford to keep up with the curve.
If you study evolution and palaeontology, whether it is diplodocus or drosophila, hadrosaurs or hominids, at some point you are likely to make use of, or even make your own, phylogenetic tree. These trees are really the holy grail of evolution studies, showing the relationships between species, what evolved from what and, in some cases, how long ago their common ancestor lived. They can be built by comparing similar characters in a creature’s appearance, like number of legs, or how they reproduce. Alternatively, modern evolutionary biologists use the information-rich genetic code in living organisms to make and compare many trees, ultimately resulting in one that most accurately represents the true course of evolution.
Making a tree inevitably involves a lot of number crunching, but the resulting diagram is elegant and informative. Try this general one of eukaryotic life
Here, Bacteria have been used as an ‘outlier’ to compare all the other members of the Eukaryotes. Each branch marks an evolutionary ‘divergence’ – a novel change that created that group of organisms. For instance, the invention of chloroplasts led to the all the members of the plant kingdom, just as the invention of feathers led uniquely to birds. The fewer the number of branches between two creatures, the more closely related they are. For instance, we are more closely related to cows and whales, than we are to marsupials.
So to a graphically minded palaeontologist, a phylogenetic tree is quite a thing to behold, but there is a way of making them even better. For many, more detailed trees, you may be dealing with specific species, and lots of them. Take this now-famous ‘megatree’ of all the dinosaurs:
While it is undoubtedly a breathtaking piece of work, with a striking design, its usefulness is questionable to all but the most dedicated head-tilting members of the vertebrate palaeontological community. More and more trees are appearing with more and more information crammed into them, and they are no longer the elegantly informative diagrams they once were.
But there is a growing trend to making phylogenetic trees beautiful and readable again, using silhouettes of the creatures being compared, rather than, or in addition to, their names. And hopefully this graphically gorgeous trend will continue with the launch of PhyloPic a new open database of life form silhouettes for use in phylogenetic and other applications. Here’s an example:
The open source database is encouraging submissions from registered users (registration is as easy as pie) of silhouettes of any creature, in solid black, to be used under a creative commons license. Users can search the database for the latin or the common name, and download the image in a variety of sizes and manipulable formats.
At the moment, the search and browse facilities are still a little clunky, and the database is rather sparsely populated with some odd looking silhouetted. What on earth are these?
They are, in actual fact (from left to right): a single-celled symbiotic euakryote, a placozoan, a human baby, a choanoflagellate, and a pterosaur. Perhaps a little more contecxt will make these silhouettes a little less mysterious.
Needless to day, as an artist and a palaeontologist, I heartily approve of this new resource and I know I’m not alone – the young palaeo-community has got silhouetted ants in their pants with excitement over it. I will certainly be contributing some images over the coming weeks, and I encourage any other artistically minded palaeontologist, zoologist or miscellaneous scientist to help to build this wonderful database.
Browse or contribute to PhyloPic here: http://phylopic.org
Each year, our Earth Science Department in Oxford has a Christmas party which incorporates, amongst other things, a cake competition. Each research group makes a cake that is in some way related to their work. The palaeontologists have always been something of a diminutive group, and have been considered the underdogs in the department, but with the arrival of three new members this year we are growing strong. We decided to take on a mammoth task with our cake – to represent the evolution of all life, from the first cells to humans, on a gigantic spiral cake. This was the inspiration:
And, after 30 hours, 32 eggs, and 3kg of marzipan, this was the outcome:
This photo isn’t really the best, and there are lots of things I would like to draw your attention to, so I have made a lovely video for your watching pleasure. You can find it *here*.
Anyway, we were all supremely happy with what we produced, which was just as well because we didn’t even win the competition. Not that I’m bitter about that at all…. It would seem that when a cake is being judged on both style and taste, you can’t rely on a thoroughly manhandled multi-bake cake and marzipan to clinch the deal. Never mind.
I plan to hand in a less edible version as a diorama alongside my thesis. Pretty sure it will boost my approval?