800-600Mya: The First Multicellular Animals, Sponges

Some cyanobacteria evolved the ability to fix nitrogen from the atmosphere. This increased their productivity and ability to photosynthesize (and fix CO₂), possibly cooling the atmosphere.

As may have happened even earlier, the Earth underwent 2-4 episodes of severe glaciation during this period, when much of the land and sea were covered in ice. Such episodes may have lasted 10-20My each, followed by short periods of intense heat (40-50°C).

Such variations in conditions may have killed many lives. At each melt-down, the seas may have had high levels of dissolved organic compounds, and this may have favored the micro-filter feeders, especially the choanoflagellate colonies, which grew larger to form the first multicellular animals, the sponges.

Sponges force water through pores into an internal, usually convoluted, canal system, acting as a chimney sucking water upwards. Cells differentiate into choanocells with flagella, and generalized cells that make the pores, outer layer ... They are diploid, and reproduce by budding into a colony; but seasonally, they may produce motile gametes that reach other receptive sponges to produce a larva, which disperses away from its parents.

One type of sponge developed an outer epidermis: a layer of cells connected by cell junctions.

In one such group, the larvae became more prominent in the life-history of the sponge, living off the rich plankton at the sea bottom. They have two epithelia, an external cover and an internal digestive layer forming a 'stomach' cavity, in which to secrete enzymes, with a 'mouth' opening.

Some even remained as 'larvae' throughout their lives — these are the placozoans.

[Warning: All photos are of modern species which have evolved meanwhile; but they are the closest to what the originals may have looked like.]

Placozoans

Placozoans may have occupied many niches, some grazing algae from the sea floor (benthic), others floating in the sea. Some had muscle cells to change shape and a neural net for coordination.

Note: That these fossils are placozoan is mostly conjecture, not based on direct evidence.

Some floating placozoans with neurons developed sticky tentacles, used as 'fishing' lines to catch plankton — these are the comb jellies.

600-550Mya: Ediacaran

Algae superseded cyanobacteria by 700Mya, while bacterial mats and stromatolites went into decline and disappeared in 300My. As the green algae bloomed after the last ice age, so did their predators.

Radiolarians
are planktonic and have a (silica) shell (enclosed by the membrane) for protection.

Foraminiferans
are large (0.1-3mm) and benthic.

Radiolarian and foram shells are among the most beautiful shapes in nature.

Cnidarians

A tentacled placozoan developed a cell with a complicated stinging cnida (a 'harpoon' with trigger hair). It was fixed to the bottom, catching its planktonic prey with its venomous tentacles, and transferring them to its mouth/stomach.

They have well-developed muscle cells, controlled by a nerve net, to contract its body thus moving its tentacles in/out. They often bud out to form colonies, but still produce seasonal gametes and larvae.

Worm World

Another major group of placozoans developed a scavenging benthic lifestyle — these are the primitive flatworms. (They could have evolved from the larvae of cnidarians.)

They evolved into the first very successful and widespread worms, with

a front-back of 'head'-body-tail having an efficient mouth-gut-anus,
simple light-detecting 'eyes' and sensory nerves concentrated in 'head',
ability to move with longitudinal and circular muscles by peristalsis (myoglobin,...)
'blood' with hemoglobin to carry oxygen, purified by nephridia; blood clotting mechanism;
hormones for communication (e.g., insulin, cytokines),

Most were small (mm) and lived off plankton on rocks, others processed organics in sand etc.

Hemichordates Arrow worms Gastrotrichs Flatworms Ribbon worms Nematodes Kinorhynchs Arthropods Rotifers

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Namacalathus

Hemichordates (acorn worms) have a nerve along their long back and gill slits to filter feed and absorb oxygen
Arrow worms are small plankton feeders; gastrotrichs are microscopic versions of them;
Ribbon worms are benthic and may be large and carnivorous (ambush prey)
Nematodes are microscopic decomposers, detritivores, very widespread
Kinorhynchs are tiny molting worms, with an inelastic cuticle, detritivores.
Rotifers are microscopic, living like ciliates.
Arthropods were hunters with a head, antennae, eyes, and a segmented body.

Out of these the most successfull were the arthropods and the worms related to ribbon worms; the latter gave rise to the mollusk worms and the annelids.

**Mollusk** worms (Halkieria)
have a raspy tongue (*radula*) to scrape algal/bacterial mats, move with 'foot'.

**Annelids**
are segmented burrowing worms, active, filtering sediment

**Echinoderms**
are hemichordates with a hard skeleton of CaCO₃, moved by water pressure

**Chordates**
evolved from hemichordates, swam better with a 'notochord' and gills

550-500Mya: The Cambrian

With the plankton becoming ever more abundant, and the rising seas exposing new continental shelves, the sponges and sea anemones (coral) grew, many forming symbiotic relations with dinoflagellates or algae, and forming vast colonies — year after year, they built up reefs.

All the major animal groups of this period had some successful branch of plankton filter-feeders.

Lancelets, a type of hemichordate, filter-fed through gills, but could *swim* in an eel-like fashion with a flexible spinal cord.

Tunicates, or sea squirts, are essentially fat lancelets fixed to the ground (sessile).

Bristle worms are annelids with bristles to move in sand, usually with a protective tube, became widespread, from seashores, to planktonic, to deep sea vents.

Jellyfish

Some sea-anemones, the hydras, developed male/female medusae, which swim and produce sperm and eggs respectively, that then combine to form larvae.

Indeed some, the jellyfish, lead most of their lives as floating medusae, with the hydra form as a larval stage. These cnidarians were very successful at the top of the food-chain in the open sea, and still form a large part of the animal biomass.

Hydras

Jellyfish

Cube jelly even have eyes to see, and so have the ability to move and hunt fast.

Chordates

Some lancelets developed into swimming animals seeing with their eyes. They still fed through their gills and swam rapidly to hide in sand and escape predators.

Following a rare whole genome duplication event, one group developed a head with a rudimentary cartilage skull protecting a brain that processes info from a smelling nose (fore-brain), small seeing eyes (mid-brain), a tongue, and balance (cerebellum). From filter-feeding with gills there is a transition to swimming towards, and eating, dying animals.

Hagfish

Arthropods

The most successful group of animals were the arthropods. They have a chitin exoskeleton, with jointed legs, antennae, and mandibles.

Velvet wormsHallucigenia

Trilobites
eat organics on sea-floor,
roll up in a ball of spines when threatened

Water Bears
microscopic, living on the seashore, can withstand being dried out

Crustaceans
have a head-thorax-abdomen
with compound eyes.

OpabiniaAnomalocaristop predators < 1mChelicerates
are hunting trilobites

Following the evolution of the first large seeing predators (530Mya), the vulnerable prey promptly adapted with a defensive array of shells, spikes, and coverings. This is seen as a Cambrian explosion (of fossils).

Brachiopods were bottom filter feeders, shelled forms of bryozoans

Mollusks developed shells.

Sponges and flatworms survived by evolving poisons.

Several protists struck back on multicellulars, as they became their parasites in their gut or blood: many reduced in size, simplified and lost their mitochondria (for lack of O₂):

dinoflagellates (→ apicomplexans, e.g., toxoplasmosis, malaria),
chromists (→ the water molds),
fungi (→ microsporidia, with resistant spores),
euglenids (→ trypanosomes, e.g., Giardia),
amoebas,
flatworms,
nematodes (→ roundworms)

At the end of the Cambrian age, green algae and cyanobacteria made up the basal oceanic plankton. They were eaten by ciliates, amoebo-flagellates, radiolarians, and foraminiferans, which in turn were food for zooplankton: various larvae and small floating arthropods.

Further up the food-web, brachiopods and sponges filter-fed on the plankton, while jellyfish ate the larger zooplankton. Trilobites and mollusks scraped the sea-bed organic mats, other arthropods hunted actively, while an army of worms and chordates scavenged for left-overs.

Species went extinct fairly commonly, perhaps because the ecological niches were still being explored, or because the oxygen level fluctuated.