Inorder to hypothesize the last common ancestor of Metazoans, data regarding theProtozoans is needed to construct a timeline of origin and compare thedifferences between these unicellular animals and the transition tomulticellularity, which is found in the metazoans. Fossil records would beneeded to analyze physical structures of unicellular species and makeinferences about the behavior and morphological evolution of multicellular eukaryotessuch as metazoans. Data collected from cellular processes and machinery inchoanoflagellates, the closest living relative to animals, including informationabout cell division and divisions of labor has been useful in hypothesizingtheir relationship to metazoans and linking them to their last common ancestor.Specialized feeding cells only found in the choanoflagellates and the metazoan upholdthis hypothesis as well as analysis of protein sequences backed by statisticalsupport. The last common ancestor of metazoans would have also most likely beenaquatic and unicellular, similar to choanoflagellates. Recreating thephylogenetic tree to show their similarities also places choanoflagellates as amonophyletic group, and despite ambiguity of exact placement from a smallamount of nonchoanoflagellate protozoa showing no clear similarities tometazoan cells, it is sufficient enough to claim they have a closer relationshipto the metazoan than with other groups.
The uncertainty of placement is partlydue to the possibility that choanoflagellates may be degenerate porifera,meaning that metazoans last common ancestor would actually be the sponges. Todetermine this, one would need to examine whether choanoflagellates divergedbefore animals originated or if they instead evolved from the porifera. Indepth studies of these relationships can be supported through mitochondrialgenome sequencing. 2. Adaptive radiation describes anevent in which a lineage diversifies quickly and evolves different adaptationsin the new lineages that are formed. This diversification and evolution of newadaptations allows for new niches to be taken up by new organisms.
In wellpreserved fossil records, morphologies and the numerous new varieties of speciesthat develop from adaptive radiation events can be examined. The Cambrianexplosions is an adaptive radiation because it introduced many new species andniches that were not seen before it occurred. During this time, new body plans,types of symmetry, patterns, and ecological opportunity evolved. This includes adaptationssuch as coelomate bodies, bilatarian symmetry, triploblasty, and newpredator/prey interactions. These innovations show how animal evolution wasable to become increasingly complex and grow in diversity through coevolution.
In living animals today, these adaptations are still seen which illustrates theimportance of evolution post-Cambrian explosion. Pre-Cambrian animals lackedobvious body parts such as heads and mouths, and did not have completedigestive tracts that were found in post-Cambrian animals. They also were mostlikely soft-bodied and fed through filtering processes as opposed to activepredation.
These differences in animals pre- and post- Cambrian explosion depicthow drastic the adaptations in animals can be in response to rapid environmentalchange, and how complex they can become over a short period of time. 3. The success of a phylum depends on theirspecies diversity, abundancy, ability to take up various niches, and their capacityto inhabit as many environments as possible. The phylum Arthropoda fits thesecriteria because it consists of over one million species, is considered themost diverse, and have adapted to survive successfully in land, water, and airenvironments. The show a great amount of resilience to extreme environments dueto the development of exoskeletons supporting an overall large size range, andhave exploited a vast number of niches with a minimal amount of competition forthe same resources because of their ability to grow through distinctivedevelopment stages. Their high rates of reproduction and motility allow them tohave an immense survival advantage over other phyla. Their fossil record showstheir acquisition of flight to be dated about 300 million years ago, meaningtheir success has be ongoing for a long time.
This adaptation alone as anextremely efficient means of transportation has given them the ability toescape predators, expand to different habitats, and make use of new resources,all necessary to maintain their position of biological dominance over other phyla. 4. Parasites have a large diversity andare able to evolve rapidly partly due to their short generation periods andlarge populations.
Animals that evolve to parasites are thought to have comefrom originally free-living and mutualistic organisms but later becamedegenerate. The concept of parasitism lies in the fact that these organisms occupya narrow niche, gaining nutrients from their host while causing them harm, butnot necessarily always killing them. Mutations in what used to be benign formsmay have led them to becoming disease-causing, resulting in counter-adaptationsfrom their host, which in response increased their resistance as well. This adaptivegenetic change is known as host-parasite coevolution, which is thereciprocation of selective pressures on one another leading to rapidadaptation. The Red Queen hypothesis sums this up to the constant change inboth host and parasite in order to counter each other’s adaptations and ensurethe survival of their own population. Benefits of parasitism are that they havecomplex life cycles that allow them to have multiple methods of transmissionand they reproduction potential is much greater than their hosts. Parasites canalso modify their host and act as an extended phenotype to increasetransmission rate, increase their growth, and make reproduction more effectivefor them.
Counter adaptations from the host can reduce the fitness costs ofparasites and parasitic infection can alter resource allocation for the host’sother bodily processes. 5. Cephalopods of the Mollusca phylumare considered among the most intelligent invertebrates due to the advancedcognitive abilities they have evolved. Intelligence in this case is measured byskills such as spatial learning, navigational capacities, and predation techniqueswhich are widely studied in cephalopods. Octopuses are the most acknowledged interms of cephalopod intelligence, which can be tested through a series ofcognitive tasks that require advanced thought processes such as dexterity, theability to use tools, and the capability to communicate or learn. In anoctopus, one could observe the ability to grasp and manipulate objects for locomotionor attaining food. The ability to communicate can be tested by placingdifferent backgrounds against its body to calculate the speed of changes inskin color controlled by nervous control of chromatophores, which has been donewith cuttlefish.
Observational learning of basic tasks such as placing objects orevasion tactics such as escape from enclosures can be studied to examinelearning capacities. It is difficult to compare invertebrate intelligence tovertebrate animals because their nervous systems are so different andintelligence in mainly attributed to vertebrates. Intelligence in invertebratesis also thought to be environment-dependent rather than social-dependent, whichmakes it difficult to know whether they can be tested the same way.