Interesting stuff about the science of evolution and the natural world

Birds are dinosaurs

This idea is nothing particularly new, but when I first heard it I thought it was stretching the truth a bit – birds may have evolved from dinosaurs though surely they became something quite different. But now I’ve discovered a little more, it seems the two share so much in common they do need to be considered as one.

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In fact the idea began to take form back in the early days of evolutionary thinking. In 1868 Thomas Huxley first showed the similarities between Archaeopteryx and dinosaurs and proposed a link between the two. It was widely accepted at the time, but fell out of favour in the early twentieth century. One reason was the presence of the wishbone in birds. The wishbone is formed by the fusion of the collar bones and at that time no fossil from the dinosaurs that birds were supposed to have evolved from showed any collar bones. More recently collar bones showed up in the fossil record so the theory was back on the table again.

Even more compelling evidence has come from the discovery of some dinosaur fossils that clearly had feathers. Feathers were originally thought to be the exclusive and defining feature of birds, so this discovery certainly put the cat amongst the pigeons so to speak. Or dare I say, the dinosaur amongst the pigeons. There is good evidence to suggest that some well known dinosaurs such as Tyrannosaurus and Velociraptor were feathered.

Anyway, all this has finally shown that birds evolved from dinosaurs, but what about the idea that they are dinosaurs. This becomes clearer when you look at the number of characteristics they share. Consider some bird features:

  • Possession of feathers
  • They walk on two legs
  • They have claws on their toes
  • Three toes point forwards and one back
  • Hollowed out bones form part of the respiratory system
  • They have a wishbone (exclusively amongst living animals)

All these features are also possessed by the group of dinosaurs they evolved from within. True, birds do have further exclusive adaptations, mainly associated with flying, but analysis of the large number of shared characteristics shows that they can be grouped with dinosaurs in modern classification. As I’ve mentioned in previous posts, all classification is arbitrary to some extent, but if we accept some method of grouping things then it does look like we have to merge these two together.

It’s also very illuminating to consider Archaeopteryx, that wonderfully iconic early bird.

archaeopteryx

It clearly had feathered wings that were capable of flapping flight, albeit probably rather weakly, so it was undoubtedly a bird, assuming this is the trait we now use to define birds. And yet it retained many of the more primitive dinosaur features. Unlike today’s birds it had teeth, a bony tail (a modern bird’s tail is purely feathers), clawed fingers on it’s forelimbs, many back vertebrae, an unfused pelvis and unfused foot bones. It may have had wings, but bodily it was very much a dinosaur. So if Archaeopteryx was a flying dinosaur, and Archaeopteryx was also a bird, then birds are dinosaurs.

I guess it shouldn’t be such a difficult thing to accept. After all, we have no problem in calling bats flying mammals, so why not? And of course, it also means that dinosaurs never became extinct. Jurassic Park is in your garden right now.

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OK, so this statement is completely wrong, but it serves to illustrate the false conclusion of thinking that we are descended from chimpanzees because we evolved from an ape of some sort and they are our closest living relative. But we evolved from them no more than they from us.

What we do share is a common ancestor which we both descended from, and this makes chimpanzees our closest living cousins in species terms, but not our actual ancestors. Understanding common ancestors is vital to understanding evolution and classification, so lets go back in time to trace our little corner of the tree of life.

DNA analysis has shown that the ancestor linking us with chimps existed about six million years ago, and other evidence suggests it was certain to have lived in Africa. No fossil has been found that can be definitely attributed to it, but there are a few contenders. So, since we have no proper name let’s just call it ape A. But what did it look like? It would have been fairly close in appearance to a chimpanzee and it would have walked on four legs (using knuckle walking similar to chimps and gorillas), lived in the forest and been an excellent tree climber.

Around this time the climate began to cool a little and become dryer, meaning the forest in Africa began to thin out and in places open plain took over. This may have caused the population of ape A to become split into two, and therefore each group pursued it’s own independent evolutionary path. One group remained in the forest and so there were few external pressures for it to change – it would eventually evolve into chimpanzees. The other group must have come under some environmental pressure to cause it to change. This may have been a further thinning of the forest it was living in, but for whatever reason, it gradually evolved to become more adapted to living in the plains rather than the forest. This branch eventually became modern humans.

So ape A is the common ancestor we share with chimpanzees and makes them our closest living cousin within the animal world. It also means that the chimpanzee’s closest cousin is us, rather than any other ape. We have both had the same amount of time to evolve since ape A existed, but the reason we look so dissimilar is due to the different path that the environment led us to take – we took to the plains and became hunter gatherers instead of staying in the forests.

The next nearest living relative to us (and chimpanzees) is the gorilla and we need to go back 7 million years from present to find the ancestor we all share – call it ape B. Again it would have lived in the African forest and there would have been a split in the population somehow, one line leading to gorillas and the other to ape A, and subsequently to chimpanzees and us.

Going back even further, about 18 million years from present, we come across the common ancestor to all the great apes. Great apes comprise gibbons, orang utans, gorillas, chimpanzees and humans. They are a separate group from monkeys and the key difference is that apes have no tail whereas all monkeys do (though in a few it is almost reduced to nothing). This is a good illustration of how common ancestors are so useful in defining evolutionary paths and modern classification. If apes are defined as separate from monkeys due to their lack of tail (although it’s actually slightly more complicated than that), then this could be traced back to the one single species that lacked a tail and is ancestor to all subsequent apes.

All major groupings will have a single common ancestor at some point. So for example the first mammal species I talked about in a previous post would have had mammary glands, three bones in the ear, warm blood and fur – and this is why all mammals possess these and why we put them into a group separate from all other animals. As we go back in time there will be common ancestors to larger and larger groupings, until we reach that one first originator of all life with that one huge characteristic that all life shares – DNA.

oak

Researching family history is very popular these days. With a bit of luck you might be able to go back a few generations and maybe find out who your four times great grandparents were and what they did. Going back any further begins to get a bit tricky, but I guess it’s possible to indulge in a bit of imagination. We could think of our ancestors from a thousand years ago working the land to put food on the table, or from 20,000 years ago, before farming, living as hunter gatherers with very simple tools. At around 100,000 years ago, our ancestors are a relatively small population existing only in Africa, and amongst them will be our approximately 5,000 times grandparents.

Now we get to the bit that needs the mind to take a slightly bigger leap in imagination. If we could find our 250,000 times grandparents they would be apes, also living in Africa, some 6 million years ago. OK, granted this is nothing new – ever since the famous cartoon of the head of Darwin on the body of an ape, from 1871 – but it’s quite sobering to remind ourselves of the closeness we share with the rest of the natural world, and the fact that we are not so different from other animals.

So we can keep going back through our direct ancestors – to a primitive primate, then the first shrew-like mammal, an early reptile (or synapsid, to be correct), the first land vertebrate, a fish, a marine worm, a very simple multicellular animal, a single celled animal and right back to bacteria.

But with ancestry, you’re not restricted to direct ancestors, you could also consider other branches to your family tree – your cousins. First cousins share the same grandparents, second cousins great grandparents, and so on. Using these links we can begin to find our relationships to other animals not within our direct ancestry. But now comes the bit that gets a bit more weird, to me anyway, but still doesn’t cross the line into fiction. If we go back to a linking ancestor way back in the time of single celled life and follow a different lineage to the present we would find that, for example, our cousin was an oak tree – in fact all oak trees and all plants. It’s impossible to know the correct numbers but I guess it could be something like a 100 billionth cousin many million times removed. And this means an actual cousin, not using it as a metaphor for species evolution. All individual oak trees are our literal cousins, albeit somewhat distant.

I think I have a new found respect for the tree huggers.

oak2

Not much to say here really, just a bit of a plug for the Natural History Museum in London. One of it’s most treasured displays is the very first Archaeopteryx fossil which was found in Germany in 1861, giving Darwin and Wallace some timely evidence for their newly published theory. And they show the genuine fossil, not a cast.

archaeopteryx

I think I love the building nearly as much as the exhibits.

panmus

Inside and out, everywhere you look, there are carvings of creatures weird and wonderful.

museum1

Bigging up Wallace

Alfred Russel Wallace (1823-1913) certainly doesn’t deserve to be forgotten. He came up with an identical theory for the mechanism of evolution at the same time as Darwin, but how many people now would know his name?

statue1

Fortunately things are changing a little and he is being brought out of the shadows to take his place in history once again. This is largely down to the work of George Beccaloni of the Natural History Museum, London who has been campaigning to give him the credit he is due. The campaign was given a huge boost by comedian Bill Bailey’s programmes on the BBC last year when he retraced Wallace’s steps in the jungles of Borneo.

Wallace was a dedicated and adventurous naturalist and spent his life studying the distribution of life around the globe and attempting to unravel the mysteries of evolution. He had two major trips to collect and study wildlife. The first was a four year exploration of the Amazon Basin. Unlike Darwin, he was not from a wealthy privileged background so needed to generate funds for himself and he did this by selling samples that he collected. The South American trip was a great success, but soon after they sailed for home a fire broke out in the ship and it was sunk. Luckily all on board were saved in the life boats, but a large part of Wallace’s samples and notes from the trip was lost.

His other major exploration was an eight year trip to the Malay Archipelago. It was during this trip in 1858 that his theory on evolution suddenly came to him – apparently when he was laid up in bed with a fever. He wrote up his ideas in a paper and sent it to Darwin back in Britain to see what he thought, asking him to pass it on to the eminent geologist Charles Lyell if he felt it was worthy. In Wallace’s absence, they decided to present the paper, along with some of Darwin’s writing on the same subject, at a meeting of the Linnean Society in London. Initially there was not a huge amount of interest, but Darwin was now spurred on. He had been developing his own ideas for some years but had been reluctant to publish, wishing to amass as much evidence as possible for what he knew would be a controversial theory. But now the cat was out of the bag, so to speak, and he rushed out his first edition of The Origin of Species the following year.

Wallace and Darwin deserve equal credit for natural selection in my view. Wallace was a modest man from a modest background and perhaps that is largely why he has become sidelined in history – but it’s good to see him getting a little of the limelight once again. A painting of him was donated to the Natural History Museum in 1923 and displayed on the stairs at the head of the Central Hall, but it was removed in 1971. Last year it was restored to the same position and it was unveiled by Bill Bailey.

painting

The Museum is also home to an ongoing project to digitise and make available online approximately 5000 letters to and from Wallace.

We now also have for the very first time a statue of Wallace. His grandson gave a speech at it’s unveiling at the Natural History Museum, which was performed by David Attenborough.

statue2

The statue captures the moment when he first saw what is now called Wallace’s Golden Birdwing butterfly. In his book he says “On taking it out of my net and opening the glorious wings, my heart began to beat violently, the blood rushed to my head, and I felt more like fainting than I have done when in apprehension of immediate death. I had a headache the rest of the day, so great was the excitement produced by what will appear to most people a very inadequate cause.”

Wow, he was certainly a dedicated and enthusiastic naturalist.

Evolution in action

When is a species not a species? It might seem that all living things are neatly divided into separate types and each is distinct from all others, but the boundaries are actually much more blurry than this. A fascinating example is shown by what are called ring species and there are two types of gull in the northern hemisphere that provide a classic case.

gull

Two of the gulls occurring in Britain are the herring gull and lesser black backed gull. They are quite distinct from each other. They do not interbreed with one another and they look quite different – herring gulls have light grey backs and black backs have, er well, very dark grey backs. If you now take a quick trip round the world, first you will find that in North America there are herring gulls but no lesser black backs. Then as you journey round you will notice that these gulls look progressively less like herring gulls and more like lesser black backed gulls. Adjacent populations are mixing and interbreeding as they are virtually identical. But as you go round the world through Russia and back to Europe, the population has now become fully lesser black backed gull and does not breed with the herring gulls there. As you travelled round the ‘ring’ one species ever so gradually morphed into the other until you get back to the start where they exist together as two separate species.

This is showing two interesting things. Firstly it’s a great example of evolution in action – one species is splitting into two in front of our eyes. For a dichotomy to occur a population needs to be physically divided into two to prevent genetic mixing so that evolution will take each of them off in their own independent direction. In the case of these gulls the populations have not been fully isolated so the genetic mixing that is happening over part of the range is blurring the concept of species. The other thing it does is give a superb insight into the gradual nature of evolution. Here we can see a gradual transition over a geographical range, but exactly the same thing happens over time as well. As things evolve the changes taking place are incredibly gradual – at no point does a species suddenly leap from being one into being another. It’s a little like counting. If we define 5 as a small number and 1000 as a large number, when we count from 5 to 1000, at what point does the number become large? It’s impossible to define, and so is the change from species to species in evolution.

There is a natural desire to define things and to put them in different boxes, but the nature of evolution tends to make this a difficult and rather artificial task. Another area which tends to confound the desire for strict definitions is that of grouping and classifying living things. Seeking to put everything into it’s own neat little box is always going to lead to problems – but I’ll probably come back to that in another post.