Sexual reproduction and sharing different DNA gives us a better ability to cope with the unexpected challenges in our environment that would otherwise wipe us out, says John Long.
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LONG AGO, RATHER than simply shedding or budding off a piece of themselves to create a new identical life form - a clone with the exact same DNA - two organisms got together and shared genetic material to create a more variable offspring. This was the true beginning of the sexual revolution. But exactly when did it happen? And why is this kind of reproduction so popular today?
Primitive life forms can largely be divided into those with a nucleus in their cells (eukaryotes) and those without (prokaryotes). Most obvious organisms today - the complex multicellular animals and plants called 'metazoans' - are eukaryotes, which contain nuclear DNA. Although some can reproduce asexually by budding off identical clones, most eukaryotic organisms undergo sexual reproduction: they share genetic material to produce a new generation.
Some animals such as hydras - jellyfish relatives - can reproduce sexually or bud off clones, depending on food availability. 'Sex' in the biological sense is really defined by the process of meiosis, which includes gametogenesis - when cells divide to produce gametes (such as egg or sperm cells) by halving their chromosomes.
When different individuals of the same species unite, the chromosome halves in their gametes recombine to begin making a new, genetically unique, organism. So how can fossils shed light on such microscopic and delicate processes that began not just millions, but probably billions, of years ago?
To answer this question, we need to delve briefly into the life of a truly extraordinary man named Reginald Sprigg, whose work in geology begat a whole new field of study that has since revolutionised our understanding of the early evolution of multicellular organisms.
Sprigg was born in 1919, in Stansbury, on South Australia's Yorke Peninsula. As a boy, he collected fossils and shells from his local beach and later, through a chance meeting with an old miner, became fascinated by minerals. Studying science at the University of Adelaide, he was fortunate to learn under great geologists such as Sir Douglas Mawson and Cecil Madigan, both veterans of Antarctic exploration. Sprigg, reportedly described by Mawson as his "best ever student", was inquisitive and liked to question and challenge the views of his professors at a time when it was not usual to do so.
Sprigg graduated in 1941 and was later brought on board a top-secret Australian government project searching for uranium deposits. It was wartime and a great race was underway to develop and utilise the properties of uranium. Sprigg worked on several key sites in Australia and was sent to study uranium deposits in the U.S., Europe and Britain to extend his knowledge of the geological settings of uranium-bearing ores. On his return to Australia in 1950 he was perhaps the world's most highly regarded source on the subject.
It was, however, a rich deposit of strange fossils that Sprigg stumbled upon in 1946 during his uranium fieldwork in the Ediacara Hills of South Australia's Flinders Ranges that remains his most enduring legacy.
At the time, Sprigg determined the fossils were probably of Early Cambrian age - about 540 million years old, partly because no large metazoan (multicellular) fossils such as these had ever been found in older, Precambrian, rocks. Sprigg identified the fossils as impressions of jellyfish and first exhibited them at an ANZAAS (Australian and New Zealand Association for the Advancement of Science) meeting in 1946. Recognising the significance of the very old age of these finds, he published a short report in Nature in 1948, as well as two important papers in the Transactions of the Royal Society of South Australia, in 1947 and 1949, describing various species of early jellyfish from the new site.
The true significance of the Ediacaran site wasn't appreciated until much later, however, after the work of Martin Glaessner, Bohemian-born paleontologist extraordinaire. Trained in Vienna, Glaessner fled Nazi Germany with his Russian ballerina wife during the war years and found work in New Guinea with petrochemical company Shell before ultimately arriving in Australia. He settled into a job at the University of Adelaide, where Sprigg's Ediacara Hills fossils caught his attention. In 1958, Glaessner published a paper similar to Sprigg's on new forms of Lower Cambrian (that is, lower in rock strata) Ediacaran fossils.
