PLANT EVOLUTION: ORIGIN-MODERN PLANTS - BIOLOGY - 2024

The evolution of plants possibly began with a freshwater group reminiscent of today's green algae and from this all current groups originated: mosses, club mosses, horsetails, ferns, gymnosperms and angiosperms.

Plants are characterized by being a very varied group and with a remarkable plasticity. From the point of view of evolutionary biology, they allow the study of an important series of mechanisms and unique events, for example, speciation by polyploidy in angiosperms.

Source: By Maulucioni, from Wikimedia Commons

During this evolutionary process that gave rise to this monophyletic group, innovations appeared such as the cell wall made up mainly of cellulose - a polymer made up of repeating glucose units.

The monophyletic consideration of plants implies the initial incorporation of cyanobacteria, which gave rise to the variety of plastids (including chloroplasts, related to the photosynthetic process) by endosymbiotic events. This event occurred in the common ancestor of this lineage.

Multicellularity (organisms with more than one cell) and proliferation of the apical meristem, a type of asymmetric cell division and zygotic retention, also appeared. However, the most striking innovation of plants was to colonize the terrestrial environment.

Below we will describe some general aspects of the evolution of plants and then we will delve into the origin of each group.

Evolutionary mechanisms

The evolutionary mechanisms involved in the origin of plants are the same ones that generated changes in other living groups: natural selection and genetic or genetic drift.

Darwinian natural selection is a mechanism of evolution that consists of the differential reproductive success of individuals. When the plants had a certain inheritable characteristic (that is, passed from parents to children) and this characteristic allowed them to leave more descendants, this morph increased in the population.

The other mechanism is gene drift, which consists of the random or stochastic variation of allele frequencies.

In the same way, coevolutionary processes with animals played a fundamental role. Like pollination and seed dispersal in angiosperms.

Evolution of plant body plans

The change in plant body plans is related to fundamental variations that occurred in the sporophyte and gametophyte phases. The studies carried out suggest that the haploid phase was previously more complex.

However, this trend underwent a reversal: the gametophyte began to be the most conspicuous phase. This phenomenon drastically increased morphological diversity from the Devonian.

Geological periods of the appearance of each group

The fossil record provides us with an incredible source of information about the evolution of all living organisms, although it is not perfect and it is not complete.

Bryophytes, very simple organisms lacking vascular tissues, have been known since the Ordovician.

The oldest strata consist of very simple structures of vascular plants. Later, in the Devonian and Carboniferous, an immense proliferation of ferns, club mosses, horsetail and progymnosperms occurred.

In the Devonian the first seed plants appeared. Thus, the Mesozoic flora consisted mainly of individuals reminiscent of gymnosperms. Finally, angiosperms appear in the Cretaceous.

Evolution of algae

Algae are the most primitive group. To them corresponds the origin of the first photosynthetic cell due to an endosymbiosis event: a cell ingested a bacterium with photosynthetic capacities.

This has been a fairly accepted explanation for the evolution of chloroplasts, and it also applies to mitochondria.

Evolution of bryophytes

Bryophytes are believed to be the first organisms to diverge from algae. They correspond to the first group of land plants. Its main characteristics are the lack of a root system and a vascular system composed of xylem and phloem.

Bryophytes dominated prehistoric environments for the first 100 million years from the origin of plants.

Evolution of vascular seedless plants

Vascular seedless plants

Vascular plants began to diversify in the Carboniferous period, specifically into seedless vascular plants.

Like bryophytes, the swimming gametes of seedless plants need water for both sex cells to meet. For this reason, it is assumed that the first forms of vascular plants were limited to humid environments that favored reproduction.

The fossils and ancestors of vascular plants date back 420 million years. The organisms were characterized by a branched sporophyte, independent of the gametophytic phase for their growth and development. These ramifications made up for their small size - they rarely exceeded fifty centimeters.

This arrangement facilitated the production of spores and thus increased the probability of survival and reproduction.

The most primitive group of vascular plants are the modern species of lycophytes (club mosses, selaginella, and ferns or isoettes). The lycophytes developed in swamps and similar terrains, until the giant forms became extinct and the small ones we observe today.

Ferns have radiated since they originated in the Devonian, along with their relatives the hair tails and brush ferns.

Importance of vascular tissue and roots

Vascular plants - as their name implies - have vascular tissue, which is divided into phloem and xylem. These structures are responsible for the conduction of salts and nutrients throughout the body of the plant.

The acquisition of a lignified tissue allowed the plants to increase in size, compared to bryophytes. These stems prevented tipping and drooping, and could transport nutrients and water to considerable heights.

Roots, on the other hand, allow plants to anchor to the soil surface, allowing them to absorb water and nutrients. The roots also play a fundamental role in allowing the plant to reach a larger size. The root system observed in the fossils is quite similar to the current one.

The origin of the roots is still unclear in evolutionary history. It is not known whether they appeared in a single event in the ancestor of all vascular plants, or in multiple events - what is known as convergent evolution.

Evolution of vascular seed plants

In the Carboniferous, plants branched into two large groups of seed plants: gymnosperms and angiosperms.

Evolution of gymnosperms

At first, the seedlings were reminiscent of a fern. Gymnosperms are plants without flowers, which all radiated within a very similar pattern. The most common today are pines and cypresses. Some of the oldest specimens include ginkgos, cycads, and gnetales.

Evolution of flowering plants: angiosperm

Flowering plants, or angiosperms, are a plant group that makes up most of the plants that exist on planet earth. Today they are distributed everywhere, and include forms as varied as trees, shrubs, lawns, wheat and corn fields, and all the common plants that we observe with flowers.

For the father of evolutionary biology, Charles Darwin, the sudden appearance of this group represented an enigmatic event that spanned an immense distribution.

Today, the group that gave rise to the angiosperms is considered to be some group of primitive gymnosperms - a shrub-like organism. Although there is no specific candidate, some forms of life with intermediate characteristics between the two groups are suspected, from the Mesozoic and Paleozoic eras.

Historically, this transformation was accepted, since it is easy to imagine the transformation of structures capable of carrying ovules (typical of gymnosperms) into carpels. Recently, it is no longer usual to actively seek these forms of transition.

The first fossils of angiosperms (and other traces such as pollen grains) date back more than 125 million years.

The flower

The most relevant innovation of angiosperms is the floral structure. It is speculated that the primitive flowers had the morphology of a current magnolia, made up of many carpels, stamens and pieces of the perianth.

In terms of visual and olfactory stimuli, the flower represents an attractive organ for pollinators. These can be vertebrates (like birds and bats) or invertebrates (like bees, wasps, or flies). Being pollinated represents an obvious advantage for the plant: it disperses pollen much better than the wind.

Pollination was a selected event, as the more animals visited the plants, the seed production did too. Thus, any change that had positively increased visits instantly offered a great selective advantage to the individual.

For example, flowering plants that by a certain random mutation began to secrete a nutrient that attracted a pollinator had a selective advantage over their peers that lacked this trait.

In addition, the fruit also represents a reward rich in energy to the animal that consumes it. After digestion, the animal defecates and thereby disperses the seeds. For example, many birds and fruit bats play an indispensable role as seed dispersers in forests.

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