What Were the First Flowers? The Problem That Bothered Darwin

Unambiguous flowering plant fossils appear around one hundred and thirty million years ago, with pollen slightly earlier, and within roughly thirty million years they had spread worldwide and displaced conifers, ferns and cycads as the dominant vegetation across most environments. That speed is what troubled Darwin, since his account of evolution expected gradual change, and a group appearing abruptly and radiating that fast looked like a problem for it. The picture has been refined rather than overturned. Molecular clock studies consistently suggest an earlier origin than the fossils show, placing the group's beginning in the Jurassic or even the Triassic, which would mean a long period of low-diversity existence before the visible radiation, and reconciling molecular and fossil dates remains an active disagreement. Occasional claims of much older flowering plant fossils appear regularly and are generally contested, and the field has learned caution after several were reinterpreted.

Both fossils and the structure of the family tree indicate a plant unlike the showy flowers that come to mind:

• •Amborella, a shrub from New Caledonia, sits at or near the base of the flowering plant family tree and has small inconspicuous greenish flowers
• •Water lilies and star anise relatives branch off next, which is why the earliest flowering plants are frequently reconstructed as small herbaceous or aquatic plants
• •Archaefructus, a Cretaceous fossil from China, is an aquatic plant with reproductive structures on a spike and no showy petals at all
• •Modelling from living species reconstructs the ancestral flower as bisexual, with parts arranged in whorls of three and with undifferentiated petal-like structures rather than distinct sepals and petals
• •Pollen from the earliest period is small and of a type suggesting insect rather than wind pollination
• •The showy large flowers familiar today are later developments, elaborated repeatedly and independently in different lineages

Several advantages combine and no single one explains the takeover. Enclosed seeds, which is what the group's name refers to, protect the developing embryo and permitted fruit, which recruits animals to disperse seeds. Double fertilisation produces nutritive tissue only after successful fertilisation, which avoids wasting resources on unfertilised ovules, unlike the conifer strategy. Faster life cycles allow some species to complete reproduction in weeks rather than years, which suits disturbed and seasonal environments. More efficient water transport, using vessels rather than only tracheids, supports higher photosynthetic rates and faster growth. Animal pollination increases the precision of pollen delivery enormously compared with wind, which permits reproduction at low population density and reduces the wasteful pollen production that conifers require. And a higher density of leaf veins, documented in the fossil record, allowed higher transpiration and photosynthesis, which may have altered regional climate by increasing moisture returned to the atmosphere.

The phrase comes from a letter written in 1879 to the botanist Joseph Hooker, and its exact content is worth pinning down because it is quoted loosely. Darwin described the rapid development of all the higher plants within recent geological times as an abominable mystery, and his concern was specifically the abruptness of the appearance and the speed of the diversification rather than any doubt about whether flowering plants evolved. He proposed possible resolutions himself, including an undiscovered period of development on an isolated landmass, which is not far from the modern suggestion of a long low-diversity interval before the visible radiation. He also connected the problem to insects, recognising that pollinator relationships could accelerate divergence, which anticipated a substantial part of the modern answer. The phrase is now used as shorthand for any unresolved question about the group, which is broader than what he wrote, and the parts of the original problem that remain genuinely open are the timing of the origin and the mechanism of the speed rather than the fact of the transition.

Flowering plants and their pollinators have influenced each other substantially, and the relationship is more complicated than the simple story of mutual origin. Insect pollination predates flowering plants, since beetles and others visited the reproductive structures of earlier seed plants, so the group inherited pollinators rather than creating them. Bees, however, appear to have originated alongside flowering plants and diversified with them, and the specialised relationships involving matched flower and insect morphology are later elaborations. Darwin famously predicted a moth with an extraordinarily long proboscis from the shape of a Madagascan orchid, and such a moth was described decades later, which is a genuine case of a testable prediction from coevolution. Fruit and animal seed dispersal changed mammal and bird evolution in turn. The consequence at the largest scale is that the terrestrial world became structurally and chemically different, since flowering plants also produce a far greater diversity of defensive compounds than their predecessors did, which shaped the insects that ate them.

The group appears about one hundred and thirty million years ago and dominates within thirty million, which is the speed Darwin found troubling. Molecular clocks suggest a much earlier quiet origin, and reconciling them with the fossils is unresolved. The earliest flowers were small and inconspicuous rather than showy. Enclosed seeds, fruit, faster life cycles, better water transport and precise animal pollination together explain the takeover.