The Best Australian Science Writing 2015 (35 page)

Being barely a third of a millimetre long, these mites come to our notice only if they make us wheeze. They are the main cause of asthma, the most common chronic disease among Australian children and a problem for many adults. Because dust mites are less than efficient at digestion, their droppings are rich in
digestive enzymes, which are so chemically active that many people, including me, suffer allergic reactions.

Other components of dust that can irk our airways include fungal spores, flakes of skin shed by pets, and pollen. Grasses, sheoaks and other plants that rely on the wind to shift their pollen release prodigious amounts. Around the world, pollen and mite allergies are increasing. The favoured explanation is the hygiene hypothesis, which posits that our cities and homes are so clean that we imbibe too few microbes to keep our immune systems active enough to prevent exaggerated responses such as allergies. Children and adults alike need exposure to the microbe-rich dirt of rural or natural landscapes.

The good news about dust is that one constituent is not the problem it was. Australian homes now harbour less lead, thanks to a phase-out of lead-based paints in the 1970s and leaded petrol by 2002. Now that our fuels are lead-free, the main concerns are with the ultra-fine particles that bypass the filtering systems in our airways to contribute to fatal heart and respiratory diseases. Red Dawn was rich in ultra-fines, but in cities, by a large margin, cars are the main source. A study on rats showed that when these particles are inhaled some of them reach the brain, where, in sufficient numbers, they probably do great harm.

An emphasis on all that is bad about dust does it a disservice. Without dust there would be more carbon heating up the atmosphere, fewer fish in the sea, less soil on some valley floors and, importantly, next to no rain – because raindrops need particles to form on. Mineral dust and soot from bushfires, floating thousands of metres above Australia, make summer downpours possible.

Scientists are excited by growing evidence that live bacteria and fungi in the troposphere, and perhaps algae and pollen as well, act as nuclei for rain. More than 2600 species of bacteria were found in air that reached North America from China.
Penicillium mould
has been detected 77 kilometres above the ground. The sky is very alive. Hundreds of thousands of microbes can exist and breed in a cubic metre of air, although their numbers drop off with altitude. Bacteria have been found in the centre of hailstones. Scientists have proposed that by increasing cloud ice, bacteria even contribute to thunderstorm form and lightning ferocity.

The story of dust is ultimately the story of everything, because the world is made of and influenced by dust. Our planet formed from coalescing dust and gas, so that as children of the Earth we are all made of dust. Cosmologists look to extraterrestrial dust for insights into the universe. The Curiosity Rover was sent by NASA to Mars to see if Martian dust would disclose proof of past life. Stardust was a space probe that collected dust from comet Wild 2 in the hope that comets preserve primordial dust from early in the life of the universe.

Our world reveals itself as a very different place when, in all its forms, dust becomes the focus. Dust reveals the power of tiny things to harm us, help us, shape our world and teach us about ourselves. Red Dawn forced millions of Australians to grapple with the stuff, but we shouldn't need a colossal storm to remind us that dust is important.

Love bug

Messages from Mungo

Germ war breakthrough

Copulate to populate:
Ancient Scottish fish did it sideways

John Long

The intimate act of copulation is old – very old. In fact, it first evolved in ancient armoured placoderm fishes called antiarchs 385 million years ago.

Fossils of the antiarch
Microbrachius dicki
show males with large bony L-shaped claspers for transferring sperm, whereas females bore small paired bones to help dock the male organs into position.

These discoveries, published in the journal
Nature
, represent the first appearance of sex involving copulation in vertebrates. It's also the first time in vertebrate evolution that males and females appear with distinct differences in their physical appearance.

Little arms, big genitals

Microbrachius
means ‘little arms', a reference to its small paired pectoral limbs. It inhabited ancient rivers and lakes about 385 million years ago. Its species name
dicki
honours Robert Dick, an avid fossil collector in the late 19th century who first found its fossils in the north of Scotland.

The new discoveries began late last year when I was working
at the University of Technology in Tallinn, Estonia, with my colleague Elga Mark-Kurik. She handed me a box of isolated
Microbrachius
bones from Estonia to examine. I found a tiny plate, less than 2 centimetres long, which had a tube of bone attached to it that I couldn't identify.

Eventually I realised it was a clasper – a primitive vertebrate sexual organ. Claspers are found in male sharks and rays and used for copulation. It was one of those sublime eureka moments: it basically meant a complete rethinking of the evolution of sexual strategies in jawed vertebrates.

Antiarchs had never before shown any evidence for this type of reproduction. We had long assumed they simply spawned in water, like many living fishes. The new discovery meant they had the ability to copulate and therefore internally fertilise their eggs. As antiarchs are the most primitive jawed vertebrates, it meant that highly complex sexual reproduction first appeared at more or less the same time as jaws and paired hind limbs appeared.

We began searching other museum collections in Europe, Australia and the US for more evidence. Eventually some amazing complete specimens of
Microbrachius
held in private collections by UK and Dutch collectors were revealed to us, showing distinct male and female features. These were then donated to the Natural History Museum, London, so we could complete our study.

First vertebrates copulated ‘square-dance' style

It's bizarre that these tiny fishes mated from a sideways position, the male and female resting alongside each other. They likely intertwined their bony jointed pectoral appendages (arms) using the rows of hooks on their inside edge. The outside arms could have helped them manoeuvre their large claspers into the mating position. With their hooked ‘arms' interlocked, the act of copulation in these fishes somewhat resembled square dancing the do-si-do.

The female's paired genital plates bore a roughened surface, a bit like a cheese grater, for the male claspers to latch on to. Once the male's clasper was in the mating position only the tip could be inserted inside the cloaca of the female to deposit sperm.

Despite their awkward looks,
Microbrachius
were highly successful little fishes whose species are found in the UK, China and Estonia. The antiarchs could have all mated in this way, as other forms such as
Bothriolepis
show similar genital structures preserved.

Bothriolepis
was the most ubiquitous vertebrate known in the Devonian period, with more than 150 species found. It lived on every continent, including Antarctica. Such antiarchs were probably the world's first truly widespread vertebrates. We now think it likely that their movable bony arms, which facilitated copulation, were the key to their migratory success.

Placoderm sex and live birth

The new discovery follows on the heels of a string of recent finds elucidating how the oldest backboned animals mated. In 2008 we discovered the oldest evidence for live birth in a placoderm fish fossil from Gogo, Western Australia.

It showed a 3D embryo fossil attached by a mineralised umbilical cord. We named it
Materpiscis attenboroughi
, meaning ‘Attenborough's mother fish'.

This discovery was followed in 2009 by another equally unexpected find, more placoderm embryos inside another group of placoderms, the ‘arthrodires'.

The arthrodires were the most diverse clade of placoderms. Yet despite thousands of specimens in museum collections, none had shown any evidence for how they reproduced. Our discovery proved they used internal fertilisation and that males had bony claspers for mating.

In early 2014 we added to this by showing that placoderm
claspers did not develop in the same way as shark claspers, as part of the pelvic fin, but evolved more or less like an extra pair of limbs.

Placoderm claspers were at first fixed rigidly to their body plates as in
Microbrachius,
so the fish had to move its entire body around to mate. More advanced placoderms such as the arthrodire
Incisoscutum
– a group of armoured jawed fishes – evolved flexible bony claspers capable of rotating forwards and becoming erect for mating.

The rise and fall and rise again of male intromittent organs

Our new research implies something that was thought impossible in biology – that fishes went from using copulation (internal fertilisation) to reverting back to external fertilisation (spawning in water), the default primitive condition seen in jawless fishes such as lampreys.

This must have occurred when bony fishes first evolved from placoderms, as none of the primitive fossil or living species such as
Polypterus
show any evidence for internal fertilisation.

We know that coelacanths mate using internal fertilisation, even though they lack copulatory structures. Some families of advanced bony fishes, the teleosts, evolved different kinds of internal fertilisation. Guppies, for example, use a modified anal fin spine to transfer sperm, although none of these teleosts ever developed paired claspers similar to those in placoderms or sharks.

Our study concludes that the most primitive jawed vertebrates originally evolved copulation as the main way of mating then lost it early on in their evolution. Later it reappeared again and again in many different animal groups.

Snakes and some lizards have paired penises but crocodiles, tortoises, mammals and some birds all have a single penis. The Argentine Lake Duck has the longest penis relative to body size
for any living vertebrate (43 centimetres for an average-sized duck). The penis was secondarily lost in many lineages of flying birds.

We humans thus mate in a way that first evolved in crocodiles and tortoises back in the age of dinosaurs. We can thank our distant ancestors, the placoderms, for first evolving this unique method of reproduction.

Love bug

Messages from Mungo

The mind of Michio Kaku

Tim Dean

Michio Kaku has an extraordinary mind. It loves nothing better than occupying itself untangling the mathematics of subatomic strings vibrating in 11 dimensions. ‘My wife always thinks something strange is happening because I stare out the window for hours,' he says. ‘That's all I'm doing. I'm playing with equations in my head.' In his day job, Kaku works at the very fringes of physics. He made a name for himself as co-founder of string field theory, which seeks to complete Einstein's unfinished business by unifying all the fundamental forces of the universe into a single grand equation. He regales the public with tales of multiverses, hyperspace and visions of a better future built by science.

Hyperbole is part of his style. He once urged humanity to consider escaping our universe, which several billions of years from now will be in its last throes, by slipping through a wormhole. Hardly a pressing concern for today, but such proclamations lasso attention and get people to think big.

Kaku certainly thinks big in his latest book, and there's plenty of hyperbole. But
The Future of the Mind
is somewhat of a surprise. What is a theoretical physicist doing writing a book about the mind – a topic usually reserved for neuroscientists, psychologists and philosophers? As a philosopher myself I was curious to
see if a physicist could shed some photons on the problem. I took the opportunity when he was in Sydney spruiking his new book to find out.