Some of the earliest traces of animal life have been found in stromatolites. (Image: Travel Ink / Getty) Doushantuo fossiles unearthed in China bear a resemblance to animal embryos. (Image: Shuhai Xiao) A recipe for early oceans: layers of plankton and bacteria, with hydrogen sulphide underneath. (Image:David Doubilet / NGS)

See Timeline: The evolution of life

WHEN Darwin unveiled his theory of evolution, the earliest known fossils lay in rocks belonging to what Darwin called the Silurian age. Older rocks seemed devoid of fossils. The apparently sudden appearance of sophisticated animals such as trilobites did not fit in with Darwin’s idea of gradual evolution.

“If my theory be true, it is indisputable that before the lowest Silurian stratum was deposited… the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer,” Darwin wrote in the first edition of On the Origin of Species. His conundrum is known as Darwin’s dilemma.

Of course, we have since discovered innumerable fossils from far earlier periods. Rocks as old as 3.8 billion years contain signs of life, and the first recognisable bacteria appear in rocks 3.5 billion years old. Multicellular plants in the form of red and green algae appear around a billion years ago, followed by the first multicellular animals about 575 million years ago, during the Ediacaran (see “The rise of animals”).

Perplexing questions

Even so, many perplexing questions remain. Why did animals evolve so late in the day? And why did the ancestors of modern animals apparently evolve in a geological …