The Sexual Revolution

Fossils from Loch Torridon in Scotland is the oldest confirmed
eukaryote fossils on Earth at 1.1 billion years old

1.2 billion years ago the first eukaryotes evolved, and with them came a wave of cellular complexity. Earth's ecosystems were transformed by a new class of multicellular organism
s, powered by mitochondria unimaginable in the bacteria and archaea over 3 billion years previously. While life forms with bilateral symmetry, legs, organs and nervous systems seen in the myriad of animals and plants today, would only evolve 700 million years ago.

Something transformed the eukaryotes seemingly overnight: sexual reproduction. This re-shuffles the genetic pack, gives rise to diversity, complexity and accelerates the pace of evolution in eukaryotes. It seemed the perfect trigger for the sudden and dramatic changes seen in the fossil record 700 million years ago. Yet a study, conducted by William Ratcliff and Michael Travisano from the University of Minnesota, demonstrates that sexual reproduction may have evolved at the same time as multicellularity, explaining why almost all complex organisms use single-celled gametes for reproduction.

One of the multicellular algal colonies 'evolved' by Ratcliff and Travisano

Ratcliff and Travisano have a reputation for their novel experimentation. They take single-celled microorganisms, such as algal cells or yeast, and make them behave like multicellular organisms. The single cells form colonies able to take on a variety of complex organised forms with the constituent cells working together towards a common goal. Some of these displayed a form of morphological evolution in response to selective pressures. In short, Ratcliff and Travisano give us a glimpse as to where multicellularity may have come from.

Further examination of the colony showed that motile single cells broke off from the cluster at regular intervals. These in turn reproduced to form multicellular colonies of their own. Mathematical models of this propagation process returned surprising results each time: that single cells due to sheer numbers would prove to be more successful in the long run. This is similar to the way corals, humans and indeed most eukaryotes produce millions of sperm cells or a biological equivalent to increase the chances of survival.

Today almost all eukaryotes use single cells, such as sperm, to reproduce

'Until now, biologists have assumed that this single-cell bottleneck evolved well after multicellularity, as a mechanism to reduce conflicts of interest among the cells making up the organism. Instead, we found that it arose at the same time as multicellularity. This has big implications for how multicellular complexity might arise in nature, because it shows that this key trait, which opens the door to evolving greater multicellular complexity, can evolve rapidly,' said Ratcliff. Indeed it is likely that multicellular organisms owe their success to the early appearance of sexual reproduction.

While genetic and fossil evidence is required to demonstrate conclusively that the two traits evolved together, Ratcliff and Travisano's work offers a dynamic perspective on the origin of multicellular organisms and one, which in some ways, seems to fit better into the story of life on Earth. As knowledge of life during the Precambrian has increased, the major evolutionary events in the history of eukaryotes have been drawn closer together.