Locomotion

How did single-celled organisms move themselves?

They used cilia for self-propulsion (particularly protists) (the last common ancestor of the crown eukaryotes had cilia)

• Rowing = cilia perform a power stroke and a lighter recovery stroke to create net thrust

• Undulation = a transverse wave moves along the cilium, drag forces are resolved into a parallel component and a perpendicular component (perpendicular > parallel), producing thrust

These movements are powered by dynein proteins, which cause microtubule sliding within the cilium

Cilia close together naturally coordinate into metachronal waves, which improve efficiency (and no need for chemical or electrical signals) - the coupling (movement of one cilia influencing the movement of the nearby cilia) is caused by the movement of surrounding fluid

How do multicellular organisms move?

In multicellular organisms, cilia is less effective due to reduced surface area to volume ratios e.g. in sponges, ciliary propulsion works for tiny sponge larvae, but cilia in sponge adults are only used to generate feeding currents (water currents that bring food particles)

Some sponge adults undergo whole-body contractions, likely powered by actin/myosin interactions in the pinacoderm
These contractions can clear sediments and in some species, they can be triggered by glutamate (possibly released by ciliated sensory cells in the exhalant pores)
The sponge will reinflate due to tissue elasticity and water currents generated by choaonocytes

Many eukaryotic cells use the actin-myosin system (including amoeba) - for this to work, cells must not have a cell wall

This shows that coordination without a nervous system is possible

What is the benefit of a neural system compared to a non-neural system?

Some animals, like sponges and placazoans will use chemical signals (hormones) to coordinate movement - this may represent a proto-neuromuscular system

As animals evolved, neural projections may have developed

• Division of labour model (leads to the specialisation of cells)

• Improve the coordination of a group of effector cells

• To increase surface area for chemical release (part of the chemical brain hypothesis = early nervous systems evolved from simple chemical signalling networks) - once these projections grew long enough, they evolved synapses to improve coordination

These required cells without cell walls

Chemical brain hypothesis = early nervous systems evolved to allow faster and more coordinated movement

Neural benefits over non-neural equivalents

• E.g. some sponge larvae use many ciliated photoreceptor cells for phototaxis, while others use only two photoreceptor cells which make synaptic contact with ciliated cells (this neural setup is more efficient)

How can we describe locomotion in cnidarians?

Early cnidarians probably had a ciliated larval phase (the planula) and a sessile adult phase (polyp)

Adult polyps have neurally controlled muscles made of actin and myosin, coordinated by a diffuse nerve net that allows precise and rapid muslce contractions (some anemones can swim using these, even though they mainly aren’t used for locomotion)
The re-extenstion of contracted muslces is provided by elastic recoil of the mesoglea or by contraction of other muscles (this only works if the mouth is closed, the gut contents can act as a constant-volume hydrostatic skeleton)

The box jellyfish (cubozoa) represents the most advanced cnidarian neuromuscular system (nerve net plus a nerve ring linking sensory structures - rhopalia, which have complex eyes and balance organs which enable control of swimming speed and direction)

Ctenophore (comb jellies) propulsion is driven by many fused cilia (comb plates)
Genomic analysis suggests that ctenophores may be the most basal extant animals, which would suggest that nerves and muscles evolved twice independently in animals, or that sponges lost them (the unique ctenophore nervous and muscle systems support the idea of independent evolution of neuromuscular systems in this group)

What is notable about Bilateria?

They are tripoblastic (they also have a mesoderm) and have a bilateral symmetry system (this makes course control much easier), but the benefits of bilateral symmetry is consistent with radial symmetry too

What is Urbi and what are its similarities to current bilaterians?

The last common ancestor of Bilateria is Urbilateria (Urbi)

There are similarities in developmental genes across different bilaterians

The Hox gene cluster of the Bilaterians (which patterns the front-to-back axis) was likely present in Urbi as well

Morphogens act upstream of Hox genes e.g. Wnt patterns the front-to-back axis, while BMP patterns the dorso-ventral axis (these act like a developmental GPS, they tell cells where they are in the body)

Was Urbi segmented?

The three major segmented groups = chordates, arthropods, annelids
Different parts of their bodies are segmented and they are far apart on the evolutionary tree which suggests that segmentation may have arisen independently
But, the developmental process for segmentation is similar across these groups - the segmentation clock
^This refers to an oscillating morphogenic signal which causes the pinching off of embryonic somites
Wnt establishes the wavefront where new somites are pinched off, Notch and Delta are involved in when the segments are made

The segmentation clock suggests that segmentation of these three groups could have evolved convergently

Developmental genes are coopted to perform similar roles in different contexts (deep homology), so Urbi segmentation system could be an ancient system that evolved through deep homology in these three groups

Even animals that don’t appear as if they are segmented (e.g. flatworms) have shared repeated patterns of some organs (especially their nervous systems) which may relate to the evolution of metachronal waves for movement in bilaterians

Urbi probably wasn’t fully segmented, but it had the genes for segmentation (deep homology)

What does the relationship between cnidarians and bilaterians suggest?

Cnidarians also use Wnt and Hox genes (similar to bilaterians), but their bodies are patterned differently

Cnidarians have an oral-aboral axis and a directive axis (we could assume that the cnidarian oral-aboral axis is homologous with the bilaterian front-to-back axis and that the directive axis is homologous with the bilaterian dorso-ventral axis, but there are different expression of Hox genes which might suggest something different)

Cnidarians lack regular segmentation of their nervous system - which fits with their lack of metachronal waves

Their repeated body parts are arranged around their body, not along it

The cnidarian larvae Clava multicornis shows early signs of bilateral features (bilateral features may have began evolving before the bilaterian group fully emerged)

In cnidarians, repeated body parts must be the same for movement to work well, whereas in bilaterians, repeated parts can change and specialise for certain movement
This allowed bilaterians to evolve many different and complex body parts

This suggests that bilaterians and cnidarians share a common ancestor (not Urbi), and then they evolved divergently with bilaterians’ repeated parts giving them more flexibility for movement

What else do bilaterians include?

The coelom (a body cavity) which acts as a hydrostatic skeleton and allows longitudinal and circular muscles to work together to perform peristaltic locomotion (a wave-like movement e.g. used for burrowing in earthworms)

What can we say about nematodes?

They have a pseudocoelom but no circular muscles (this makes movement seem impossible because they can’t re-extend the longitudinal muscles once they’ve contracted)

The fluid in the pseudocoelom is under high pressure, so when longitudinal muscles contract on one side, the body bends allowing undulatory movement (instead of peristalsis)

Chordata also evolved undulatory movement, but instead of a pressurised body cavity, they evolved the notochord and they have segmented muscles outside the notochord

Undulatory movement evolved independently in nematodes and chordates, but they use different structures (a pseudocoelom, and a notochord and segmented muscles)

How can we describe the movement of vertebrates underwater?

Chordates will use undulations to swim (while nematodes use it to crawl)
These movements are different to how tiny protists use cilia to swim because fluid dynamics change with size

• Small animals have a low Reynolds number where viscous forces dominate

• Larger animals have a larger Reynolds number where inertia matters more - cruising and streamlining is important

Fish generate forward motion by pushing backwards with their body waves (Newton’s third law) - it is more energy efficient to push a large mass of water slowly than a small mass of water quickly (deeper tails are more common)

Fish anatomy for locomotion

• Vertebrae replaces the notochord for strength

• Streamlined bodies and crescent-shaped fins

Undulatory movement in chordates also works on land which likely helped with their transition to life on land

How can we describe the movement of vertebrates on land?

The tetrapods (a group of lobe-finned fish)are the only group of vertebrates that completely transitioned to life on land - their limbs evolved from lobe-fins (by adding digits and losing fin rays)
This may have teaken place underwater e.g. the early tetrapod Acanthostega still had gills and a fish-like tail (it was primitively not secondarily aquatic)

Undulatory movement continued to be used on land (e.g. lateral undulations increase stride length and therefore speed)
Two tetrapod lineages (archosaurs and stem mammals) abandoned undulations in favour of an upright posture
When the legs are under the body, lateral undulations are no longer useful, but stride can still be increased by

• Standing on tiptoes (transitioning from plantigrade to digitigrade or unguligrade postures)

• Elongating distal limbs

• Moving with dorso-ventral undulations e.g. for galloping
  Mammals often used dorso-ventral undulations, while archosaurs were more limited, which could explain why bipedalism evolved more often in archosaurs

Return to water has occurred independently using adaptations such as limb-based paddling, body and tail undulations, or flying underwater e.g. cetaceans (dolphins/whales) have horizontally wide tails for dorso-ventral swimming (indicative of their land ancestry)

Some vertebrates (e.g. primates) moved to trees and evolved opposable thumbs to help grip branches, which shaped early human evolution