Until now: The last known supercontinent had broken up, opening up the Atlantic and Indian oceans. Dinosaurs were rampant on the globe, on land, in water and in the skies. Mammals were growing but remained tiny. Ferns and seed-bearing gymnosperms like pines were the only kind of plants to have colonised the land.
Plant reproduction evolved surprisingly slowly. For the longest time, flora reproduced individually and asexually through spores. Spores are unicellular units released by the millions. They appear to the naked eye as coloured dust or smoke. Under the right conditions, these spores produce a new organism on their own through mitotic cell division. But when the Cambrian Explosion (when evolution got on steroids) happened, the first seeds appeared. Seeds are gendered: male- and female-type seeds grow on plants and fertilise each other. They’re different from the seeds we usually see today, which are enclosed by a fruit. The exposed seeds that dominated plant-life until the Triassic period were called gymnosperms. They developed and develop even today on the surface of leaves or bark. A typical gymnosperm is a pinecone. The cones are either male or female and can be produced by the same plant. The female cones are bigger.
The male cones produce pollen that are blown by wind and get deposited on the female cones. Fertilisation then happens and seeds develop, which are once again sent blowin’ in the wind. As it happens, seeds also coevolved with insects, which might seem surprising at first but makes sense when we realise that insects pollinate and so can disperse seeds very effectively. Several insects like weevils and other bugs feed on seeds. In the process, they tend them to the ground. Moreover, undigested often seeds pass through an insect’s system and get deposited in locations farther away from their source. To take advantage of these clumsy but rewarding schemes, plants evolved other means to attract insects. The seeds of the cadaghi plant, for instance, contain a resin required by meliponine bees for building their hives. The seeds are sticky and stick to the insects’ bodies when they collect the resin. Once the insects deposit the resin, they discard the seeds. Because bees and other flying insects have been known to travel up to a few kilometres, this technique of dispersal is especially useful.
Ants are also awesome when it comes to seed dispersion. They collect large quantities of food and have that spartan discipline to be able to carry heavy stuff over long distances. Since they favour sugary and fatty substances, seeds evolved to produce them such substances. The insects then began to collect these seeds and carry them with their mouths. Several seeds got dropped on the way. Many more were eaten; some of the ants failed to digest them properly. Ultimately, most seeds of the seeds got stored in ant colonies.
And in turn, several other insects have exploited this behaviour of ants. Stick insects lay eggs that look quite sugary, and ants dutifully carry them into their colonies. There, the insects hatch and many of them behave much like ants in the first few days – before they escape the colony. Just like plants needing their seeds to be dispersed widely, animals also need their species to be spread over large distances for survival of the species. Thus, seeds and insects helped each other evolve through history.
Then, one day some 250 million years ago in the Triassic period, as tiny animals with elongated tongues and large dinosaurs with spiny scales roamed Earth, a flash of colour appeared. A plant grew a new form of reproduction that promptly attracted all kinds of insects and other small animals. The first flower on the planet had appeared.
Flowers didn’t grow much during the Triassic but began to spread around a little during the Jurassic period, which lasted from about 200 million to 145 million years ago. And as the dinosaurs evolved, so did flowers. Turns out, flowering plants (or angiosperms) were much more suited to insects.
Flowers contain both male and female reproductive parts but have developed mechanisms that reduce self-pollination – like having spread out anthers (male parts that produce pollen) so that the distance from the central stigma (female part that receives pollen) is increased. Insects pick up pollen for a variety of purposes and help in fertilising the plant by dropping male pollen on a flower’s female parts. In the process, a fertilised flower grows to become a fruit and the fruit encloses the seeds.
While plants perfected this multi-layered mechanism of reproduction, the dinosaurs were becoming bigger and more diverse. And it wasn’t just them that were growing in size. Oceans teemed with gigantic plesiosaurs and pliosaurs. Ichthyosaurs had reached the pinnacle of diversity in the waters. Crocodiles made the transition from land back to water. Several new types of reef-building organisms formed. The plankton diversified suddenly as well.
It wouldn’t be far off the mark to say almost all land was covered with dinosaurs. The herbivores dominated among then, with the long-necked brachiosaurus, brontosaurus, diplodocus and apatosaurus consuming everything green and tall enough. The medium-sized stegosaurus and ankylosaurus are away the shrubbery. There were plenty of carnivorous dinosaurs as well. Finally, there were quite a few mammals as well although they remained tiny.
The biggest evolutionary milestone that was crossed in the Jurassic period was the splitting of the birds and the reptiles as distinct from one another. Among the first reptiles were the cold-blooded salamanders; the birds were warm blooded. That is not to say that there was nothing in the sky before this time. As we saw in the previous instalment of this series, there were many flying dinosaurs – just that they weren’t technically birds. Some of the largest and the smallest creatures flying around existed during the Jurassic.Of the flying proto-reptiles, the pterosaurs were the most common, followed by the furry, dog-like anuroganthus that had three fingers and wings growing out of the fourth. But the first bird ancestor, classified as the link between flying reptiles and today’s birds, was the tiny pigeon-sized archaeopteryx that about 160 million years ago.
As time progressed, flowers blossomed, and feathered birds grew in number, our planet entered the Cretaceous era some 145 million years ago. The continent of Pangea had now fully broken up into Laurasia and Gondwana. By the mid-Cretaceous, Laurasia and Gondwana had also broken up in turn to yield all of our present-day continents but in a different arrangement. Life burgeoned. As plants evolved to disperse their seeds wider, so did the insects multiply rapidly. A positive feedback loop took hold that allowed new types of insects to mean plants soon possessed new kinds of flowers, of all shapes and sizes. Though flowering plants had already been becoming more common, they didn’t really grow in variety or size till a very special flying insect appeared, the one we think of when we say the words pollen, nectar or honey. And for good reason.
The bees are a magnificent genus. There are over 20,000 species of bees today. They are the only flying insects to be used for commercial pollination. When the first bees came, they started collecting a sweet substance some flowers produced for food. In the process, the pollen would get stuck to the bees and then get deposited on other flowers’ female parts as they’d go about their business. To get onto this wagon, flowers started to produce more pollen and nectar. And in turn, bees responded by developing hairy bodies that could collect pollen better: more pollen meant more nectar. The flowers then responded by becoming more colourful. The bees went a step further to develop combs and baskets on their bodies with which they could collect and transport pollen effectively.
This working relationship between the bee and the flower became so symbiotic that both now have an established working order. Flower nectar is sweet and provides food for the bees. So the rest of the plant evolved to produce sugary stuff for the bees’ benefit, like tree resin. Some plants also produce soft leaves that can be cut by bees and used in their colonies. In turn, bees established a societal hierarchy: the queen that lives in the colony and lays eggs; the drones whose only job is to mate with queens from different colonies at mating sites; and the all-female worker bee crew that went around pollinating flowers and collecting food for the colony.
The common phrase “birds and bees” has become a metaphor to teach children about sex and sexual reproduction because pollination and egg-laying are easy to understand and clearly distinct from human sexual reproduction. It makes for a beautiful and serendipitous coincidence that both birds and bees actually appeared on the planet together. While the Cretaceous period is most known for bringing an end to the most massive creatures on the planet, it also gave us the smallest and the most useful insect known to humans.
In the meantime, the birds had started to diversify in the skies. Feathers were everywhere, and proving to be useful. Colourful plumages attract mates. And the ability to moult and shed feathers periodically meant that feathers could change colours according to seasons, offering better camouflage. Further, feathers provided much needed temperature regulation. For animals that needed to maintain body temperatures, feathers were and are insulation. For those that needed to shed heat after a strenuous hunting exercise, it offered the capability to.
In fact, the evolution of feathers was so impactful that dinosaurs on land started growing them, too. A common example is the famous velociraptor. When Jurassic Park III (2001) was made, the physique of the velociraptor (or just “raptor”) was not well understood. It was depicted as being hard-skinned, scaly, smart and large, often hunting and working in groups. However, research since then has shown that raptors were small, no bigger than a house cat. And like the cat, they were solitary hunters and did not work in groups. Velociraptors weren’t smart, either: had a door with a handle and two children on one side existed during the Jurassic or Cretaceous periods, a raptor would never have figured out how to get inside. The name ‘velociraptor’ literally translates to “speedy thief” in Latin but the raptors had stout legs and probably ran as fast as a kitten. Their method of attack seemed to be to slash or stab a prey with a large talon or claw and then retreat to a safe distance till the prey bled to death.
But the most important finding of all was that the raptors were feathered. Researchers even believe that they were probably warm-blooded, like today’s birds.
What a happy picture the Triassic and the Jurassic painted.
Meanwhile, a very large space-rock was hurtling towards Earth.
The next instalment of the series will discuss the most recent and most well-known mass extinction: the one that wiped out dinosaurs entirely and helped humans emerge as the dominant species.
Sandhya Ramesh is a science writer focusing on astronomy and earth science.
Note: This article was updated on November 9, 2016, to clarify that lizards and snakes belong to the reptile family and that the pterosaurs were flying reptiles that were slowly replaced by birds.