Posts Tagged ‘evolution’

Welcome to part 2 of the Arthropods special, and today I’m giving you a whistle-stop tour of the myriapods. This group includes centipedes and millipedes (as well as a couple of less important relatives), with approximately 12,000 species currently known.

Centipedes and millipedes are common enough if you look through leaf litter or under stones and flowerpots in the garden. What’s the difference between centipedes and millipedes? Well, a common mistake is about the number of legs (i.e. 100 for a centipede and 1000 for a millipede – this isn’t true). The number of legs in a centipede varies between 20 to 300, and in millipedes ranges from 36 to 750.

The easy way to distinguish between a centipede and a millipede is to look for the number of legs per body segment. A centipede has 2 legs per body segment and a millipede has 4 legs per body segment. They also differ in terms of diet – centipedes are active hunters and carnivores whilst millipedes are detritivores (eating decaying leaves).

Centipedes and millipedes are a very successful group, and have been around on the Earth for at least 440 million years. An earlier relative of centipedes and millipedes called Arthropleura lived 300 million years ago and was able to reach lengths of 2.5m. This makes it the largest land invertebrate ever, and could grow this large due to higher concentrations of atmospheric oxygen at the time.

So, here are some interesting photos of centipedes and millipedes from around the world. Enjoy!

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Happy birthday to you, happy birthday to you, happy birthday dear Charles Darwin, happy birthday to you!

Happy Birthday Darwin!

On this day in 1809 Charles Darwin, arguably one of the most important scientists ever, was born in Shropshire. Charles Darwin is famous for his book On the Origin of the Species where he introduced ideas to explain the origin and diversity of all living species via Natural Selection and Evolution. Darwin was interested in most things, and his work as a geologist and naturalist gave him to opportunity to travel around the world on a 5-year voyage aboard the ship HMS Beagle. Keeping careful notes and making copious observations during the expedition, Darwin saw great biodiversity and it allowed to him to begin considering the origin of this. When he returned to England he began to formulate his idea of Natural Selection.

HMS Beagle

What is Natural Selection?

Darwin had noted that nearly all the species he had encountered were perfectly adapted to a variety of different habitats, diets and lifestyles. His visit to the Galapagos Islands (near Ecuador) had allowed him to study a group of birds (now known as Darwin’s Finches). He was amazed at the variety of different beak shapes and sizes, each adapted to a different way of life.

The Galapagos Islands

How did this happen? Natural selection requires three factors. The first is variation (differences) between individuals. The second is competition between organisms (e.g. not enough food to feed every organism) and finally an environmental change.

Darwin postulated that originally a group of finches arrived at the Galapagos islands from mainland Ecuador. There was variation of beak size within this group of finches. Because there were different food sources on the island (seeds, fruit, insects etc) different beak sizes were more suitable for different diets. For example, large beaks would be able to break open seeds that smaller beaks wouldn’t. If there were plentiful seeds, the larger beaked birds would find more food, have more offspring and therefore pass on the genes for the larger beak. This would continue as long as larger beaks gave a survival advantage. Eventually, with successive generations and continued ‘selection’ for a certain feature, the original population of birds diversified into many different species.

Darwin's Finches

Darwin realised that this same process, occuring over millions of years, could explain the diversity of all living (and extinct) species.

The Theory of Evolution has shaped our understanding of diversity, formation of new species and our position in the Tree of Life. So, thanks Darwin, and Happy Birthday!

The Tree of Life

 

65 million years ago, the Earth suffered an extinction event which removed around 50% of all living species on the Earth, including all non-aviandinosaurs (more on that little detail later). The K-T extinction (or recently renamed the Cretaceous-Paleogene extinction) occured over a geologically short time span (between 100,ooo and 2,000,000 years) but left almost no major animal group without some casualties. The non-avian dinosaurs are the most famous, but a whole range of other important groups went extinct.

Allosaurus - a large carnivorous dinosaur

So, what didn’t make it through the event?

Non-avian Dinosaurs – Having been the dominant group of organisms on land for the past 135 million years, the dinosaurs were a very diverse and highly evolved group. Present on every continent, they occupied very complex food webs and ecologies. They were almost certainly warm-blooded (endothermic – generate their own body heat via respiration) and this would have made them reliant on a good and steady food supply. Dinosaurs had already been in decline for approximately 10 millions years before the K-T event, but what was the last straw?

Pterosaurs– Often thought of as dinosaurs or early birds, pterosaurs were in fact a group of winged reptiles that had their heyday in the Jurassic period, but were still present right up until the end.

Pteranodon - a large pterosaur

Marine reptiles– large marine reptiles such as plesiosaurs, mosasaurs and icthyosaurs didn’t make it through.

Mosasaur - large marine reptiles that went extinct at the K-T event

Ammonites – these large marine molluscs, easily found as fossils across Britain, had been very common right up until the K-T event, and then the entire group went extinct. Their closest cousins, nautiloids, survived however.

Which groups got through?

Avian Dinosaurs – Avian dinosaurs, or birds as they’re usually called, survived the event relatively intact. There were some species that didn’t make it, but the vast majority did.

Crocodiles – a distant relative of dinosaurs, crocodiles and alligators survived the mass extinction event, probably because their feeding habits and ability to burrow a little helped them through tough times.

Mammals– Luckily for us mammals made it through. At the time mammals had yet to diversify, and would have resembled rat-sized and rat-shaped animals. Whilst some groups took a hit (for example, marsupials), on the whole the group survived well.

Morganucodon - an early mammal

Fish – the vast majority of fish survived the K-T extinction, although sharks did a little worse than bony fish.

Amphibians – Frogs, newts and salamanders seem to have gotten through relatively unscathed, although their rather incomplete fossil record makes it difficult to draw strong conclusions.

There were other groups of course, but I’ve just focused on the main ones. So, what exactly happened to cause the loss of so many species, in such a short space of time? And why did some groups go extinct whilst others were able to cling on? Palaeontologists and geologists have found evidence of three major environment changes that occur around the K-T event. How could they have caused an extinction?

The Chicxulub Impact

65 million years ago, the Earth was struck by an asteroid, hitting the ocean in what is now part of Mexico. The asteroid was approximately 15km wide and released the 420 ZJ of energy (that’s the same as 1 billion atomic bombs).

Chicxulub - map of impact

This would have caused mega-tsunamis, massive wildfires across the Americas and the complete destruction of any large living things within 2000km of the impact. The most significant event however was the large amount of sulfur dioxide and other debris flung up into the atmosphere. This would have blocked the sunlight completely for about a year, and then reduced the amount of sunlight arriving by 20% for 10 years.

Artwork of the Chicxulub asteroid impact

This would have had a huge and very detrimental effect on any plants trying to photosynthesis, and would have severly disrupted any food chains. Evidence such as the crater, global layers of iridium (a metal rarely found on Earth) and deposits from mega-tsunamis show this event definitely happened, and currently most palaeontologists think that it is the main contributor to the extinction.

Chicxulub - Gravity anomaly map showing crater, now buried under sediment

The Deccan Traps

The Deccan Traps are flood basalts (a rock formed from volcanic eruptions) from before the after the K-T event near Decca, India.

Deccan Traps - map showing location

Produced by over 800,000 years of eruptions, these rocks show that there was massive volcanic activity at the time and this would have contributed to any gradual decline, as well as hindering any recovery. Producing large amounts of ash would reduce sunlight, and therefore photosynthesis, and there is evidence that chemicals released during the eruptions (e.g. selenium) may have interfered with the production of eggs in dinosaurs). However, recent evidence has suggested that whilst the Deccan volcanism would have cause signficant problems, it wasn’t enough to cause the extinction.

Deccan Traps - kilometers of basalt from the massive eruptions

Marine Regression

Towards the end of the Cretaceous, sea levels dropped considerably and this mean that large stretches of coastline were lost, along with the species that lived there. This would have severely stressed marine ecosystems, as well as organisms that lived near the coast (including dinosaurs).

So, what happened to the dinosaurs then?

The likely scenario is that sea-level changes and massive volcanism was causing environmental stresses globally. Reduced photosynthesis and loss of habitats meant that food chains became stressed and more vulnerable. Large demanding animals, such as non-avian dinosaurs, felt the effects first and struggled to find enough food. The same went for large organisms in the oceans. If you were small, or could scavenge and get by on detritus (dead organic matter) you had a better chance. Then, when the asteroid hit at Chicxulub it was enough to add that last bit of environmental stress, and pushed many already-vulnerable groups over the edge and into extinction.

So the dinosaurs are well and truly extinct then?

Well, actually, no. You’ll notice that I’ve referred to dinosaurs as ‘non-avian’ dinosaurs, and to birds as ‘avian’ dinosaurs. Birds actually share a direct, unbroken lineage to small carnivorous dinosaurs. This is supported by excellent fossil evidence (of feathers, bone structure, sleeping posture and more) and also DNA evidence that shows that birds are dinosaurs. In fact, the classification that birds belongs to (Avialae) is actually part of the Dinosaur superorder. So, the next time you watch a crow walking across the lawn, imagine it without any feathers and you’ll see exactly how similar they are to their ancestors. So, to answer the question ‘What happened to the dinosaurs?’, the answer is they grew feathers, wings, and started chirping!

Falcarius utahensi - a feathered dinosaur

What’s the link between zebras, climate change and giant lasers?

The answer is they’re just a few of the topics discussed by some of the country’s top scientists at the GCSE Science Live! Event on the 25th November. Mr Bilton and Mrs Camm took 32 members of 11 Sci 1 to the talks held at the Dominion Theatre, London.

The event attracted around 1600 GCSE students from across the country, who had gathered to listen to five scientists discuss a range of fascinating topics.

Prof. Steve Jones, a geneticist, talked about the relative merits of the nature vs nurture argument. Prof. Sir David King, who was once the Chief Scientific Adviser to the Government, spoke about Climate Change and the problems that will need to be tackled in the future. The infamous Prof. Richard Dawkins posed the question ‘Is Evolution Predictable’ and Dr Kate Lancaster explored the use of high-powered lasers to trigger nuclear fusion reactions – a source of incredible energy. The talk was concluded by chemist Prof. Andrea Sella, who looked at the connection between chemical reactions and the patterns found in the skins and fur of animals.

The talks were delivered in a thoroughly engaging manner by scientists that conveyed their passion and love of the subject. The students really enjoyed themselves and were still discussing the lectures several days later.

The 5 scientists who spoke at GCSE Science Live!