Trends in Complexity and Body Plans

1Overview

A sponge sits on a rock and filters water. A flatworm glides along a pond bottom. A hawk dives at 100 kilometres an hour with eyes, brain and wings coordinated in milliseconds. All three are animals, yet their bodies differ enormously in organization. Looking across living things, biologists notice repeating trends in complexity: changes in body plan that appear again and again as lineages branch. This lesson follows ten trends listed in the curriculum: symmetry, coelom, tissue development, transport, gas exchange, cephalization, reproduction, vascularization, alternation of generations and seed production. It also revisits the contrasts between single-celled and multi-celled life from earlier lessons.

2Single-celled and multi-celled life: the starting point

Earlier lessons established the major contrasts between organisms built of one cell and those built of many. The table collects them in one place, since the trends in complexity build on them.

Prokaryotes were the first cellular life and dominate every habitat, so they are not "primitive leftovers"; they are successful lineages. Multicellularity added a new kind of complexity: division of labour. Once cells specialize, new problems appear that need solutions, which is why the trends below emerge: the body needs a shape, a way to organize tissues, a way to move materials and exchange gases, a way to coordinate itself and a way to reproduce on land or in water.

3Seven trends in animal body plans

Trend 1: body symmetry

Symmetry describes how body parts are arranged around an axis.

• Asymmetrical: no consistent symmetry, as in sponges.
• Radial symmetry: body parts arranged around a central axis, with a top and a bottom but no left or right. Jellyfish and adult sea anemones are examples.
• Bilateral symmetry: the body divides into two mirror-image halves along one plane. Butterflies, crabs and humans are examples.
• Secondary radial symmetry: bilateral larvae that become radially symmetrical adults, as in sea stars and sea urchins.

Trend 2: tissue development

Early in development, animal embryos form germ layers. Sponges lack true tissues. Radially symmetrical animals such as cnidarians are diploblastic: two embryonic layers, the ectoderm (outer) and endoderm (inner), with no continuous middle layer. Bilaterally symmetrical animals are triploblastic: a third layer, the mesoderm, forms between ectoderm and endoderm. Mesoderm gives rise to muscle, the circulatory system and much more, which allows larger and more complex bodies.

Trend 3: the coelom

A coelom is a fluid-filled body cavity lined entirely by tissue from the mesoderm, lying between the gut and the body wall. Animals are classified by it:

• Acoelomate: no body cavity; the space between gut and body wall is filled with tissue. Flatworms are an example.
• Pseudocoelomate: a cavity exists, but mesoderm lines only the body wall, not the gut. Roundworms are an example.
• Coelomate (true coelom): the cavity lies entirely within mesoderm and is lined by epithelium. Annelids, mollusks, arthropods, echinoderms and chordates are examples.

Trend 4: cephalization

Cephalization is the concentration of an organized nervous system, with sense organs, at the front (anterior) end of the body. It arose alongside bilateral symmetry and supports directional, controlled movement. An animal that travels head first benefits from having its eyes, chemical sensors and brain where it meets new surroundings first. Radially symmetrical animals, which have no front end, lack it.

Trend 5: transport

Very small or thin animals, such as sponges and jellyfish, rely on diffusion to exchange gases, nutrients and wastes with the water. As bodies grow, diffusion distances become too long, so larger and more complex animals depend on bulk flow through circulatory systems that move fluid to every cell.

See also the article on how the heart works.

Trend 6: gas exchange

All aerobic organisms need to take in oxygen and release carbon dioxide across a moist surface. The solutions form a sequence:

• Direct diffusion across the body surface works for organisms under about 1 mm thick, such as flatworms. Flat bodies keep every cell near the surface.
• Skin: earthworms and amphibians exchange gases through moist skin with a dense capillary network just beneath.
• Gills: thin, highly branched and folded filaments take dissolved oxygen from water into blood in aquatic animals such as mollusks, annelids and crustaceans, and in fish.
• Tracheal system: insects have air tubes that carry air through openings (spiracles) directly to tissues, independent of the circulatory system.
• Lungs: mammals have air passages ending in alveoli, whose walls are one cell thick and contact capillaries closely. Each lung has about 300 million alveoli, giving roughly 75 m² of surface area.

As animal size increases, the surface area-to-volume ratio drops (compare lesson two), so larger animals need specialized respiratory organs with a very large internal surface area. Gas exchange and transport trends evolved together: a heart with more chambers is useful only if blood is picking up enough oxygen at the lungs or gills.

Trend 7: reproduction

The trend in reproduction runs from asexual and simple sexual strategies toward life cycles that depend less on water and give offspring more protection. Nearly all multicellular organisms reproduce sexually, and purely asexual multicellular organisms are exceedingly rare. In plants, sperm that must swim to an egg tie bryophytes and ferns to wet conditions; the evolution of pollen and seeds freed seed plants from this need. In the life cycles from the meiosis lesson, animals are diploid-dominant, while many fungi and algae are haploid-dominant, and plants alternate generations.

Fungi, protists and the diversity of reproductive strategies

Reproductive strategy also varies within eukaryotic kingdoms. Fungi reproduce asexually by fragmentation of hyphae, budding in yeast, and spores made by mitosis. They reproduce sexually when two haploid cells fuse (plasmogamy) and their nuclei fuse (karyogamy) to form a diploid zygote that undergoes meiosis to produce spores. Sexual reproduction in fungi is often triggered by harsh conditions, which suits the idea that variation is most valuable when the environment is changing. Some fungi reproduce only asexually, while others use both methods.

4Trends 8 to 10 in plants: vascularization, alternation of generations and seed production

Plants moved onto land from water and faced drying out, loss of water's buoyancy and support, UV radiation and sperm that could no longer swim. Adaptations included waxy cuticles, stomata, lignin, UV-absorbing pigments and apical meristems.

Vascularization

Vascular tissue transports materials. Xylem carries water and minerals up from the soil and is strengthened by lignin; phloem distributes sugars made in photosynthesis. Vascular tissue allowed plants to grow larger. Bryophytes (liverworts, hornworts and mosses) lack vascular tissue and are seedless; lycophytes and monilophytes (ferns, horsetails) have vascular tissue but no seeds. The first vascular plants appeared in the late Ordovician.

Alternation of generations

Land plants cycle between a multicellular haploid gametophyte and a diploid sporophyte. Sporophytes make haploid spores by meiosis, and gametophytes make gametes by mitosis. Over land plant evolution the sporophyte became more prominent while the gametophyte shrank: bryophytes are dominated by a gametophyte, with a barely noticeable sporophyte, whereas in seed plants the sporophyte can be tree-sized and the gametophyte is a tiny cluster of cells supplied by the sporophyte.

Seed production

Seed plants produce two types of spores (heterospory: megaspores and microspores). Pollen grains are protected male gametophytes that resist drying and travel by wind, water or animals, delivering sperm without free water. Seeds protect and nourish the embryo, resist drying out and stay dormant until conditions suit. Gymnosperms (such as conifers) have exposed seeds and dominate some cold, dry ecosystems such as boreal forests. Angiosperms (flowering plants) enclose seeds in fruits, house reproductive structures in flowers, and are now the most abundant and diverse plant group in most terrestrial biomes; their diversity is linked to coevolution with pollinators, especially insects.

5Why biologists look for trends

Trends in body plan are not only descriptions; they are clues for working out relationships. Features such as the coelom and the pattern of early development (whether the mouth or the anus forms first at the opening of the early embryo: protostomes and deuterostomes) divide animals into major branches. Arthropods, mollusks and annelids are protostomes, while chordates and echinoderms are deuterostomes. Such features are used, with DNA evidence, to build the phylogenetic trees of the next lesson. They also help predict unseen features: an animal with a four-chambered heart is expected to have lungs and to be a bird or a mammal (or a crocodilian), and a plant with seeds is expected to have vascular tissue and pollen.

6Putting the trends together

7Summary

• Single-celled and multi-celled organisms differ in cell type, cell number, oxygen use and reproduction; multicellularity adds division of labour.
• Animal trends: bilateral symmetry, three germ layers, a true coelom, cephalization, closed double circulation and specialized gas exchange organs.
• Plant trends: vascular tissue, a larger sporophyte and smaller gametophyte, and pollen and seeds.
• Complexity does not mean better; each trend solves a particular problem and can evolve independently in different lineages.

?Quick check

Try each question first, then reveal the answer.