Period· The columns of time

Ectasian

1.4 Ga → 1.2 Ga

The Ectasian, from 1.4 to 1.2 billion years ago, is two hundred million years almost empty of recorded events, to which the appearance of sexual reproduction is usually referred.

Period of two hundred million years almost devoid of recorded events, to which the appearance of sexual reproduction is generally referred.

What the evidence shows

No division of the chart is poorer in events: the period takes its name only from the continued extension of sedimentary covers, from the Greek ectasis, and across its 200 million years it records no glaciation, no extinction and no upheaval that left a marker. What occurs within it belongs to cell biology. Sexual reproduction consists of two inverse operations, meiosis which halves the genome and fertilisation which restores it, and the genes governing meiosis occur throughout the eukaryotic tree, including in lineages long held to be strictly asexual. That distribution places the machinery no later than their common ancestor, without any method being able to fix the moment. The consequence is arithmetical: a descendant results from the recombination of two parents rather than the copying of one, and variation ceases to depend on mutation alone.

Bounds and conventions

The two round numbers bounding the period correspond to nothing. Nor is the origin of sexual reproduction dated: it is referred to this interval from the distribution of meiosis genes among living lineages and from molecular clocks, which supply a range and not a moment; the selective advantage that favoured it remains debated.