Why do we see colours?

Our vision is responsible for perceiving colours and shapes in our environment. This is possible because of a special type of cells present in the eye retina* named cone cells. These cells contain pigments that absorb light at different wavelengths of the luminous spectrum. In simple terms, these pigments can sense different colours. The presence of different cone cells with different pigments is what supports colour vision. The greater the diversity of pigments, the more colours will be perceived. When the light interacts with the cone cells, these will transmit an electric signal to the brain. The brain will combine the information from all the different cells to construct a detailed picture of the outside world.

Colour vision in vertebrates* evolved 540 Mya in an early ancestor. This vertebrate ancestor carried four types of cone cells and therefore four types of pigments. These prehistoric cone cells were able to perceive light in the red, green, blue and violet or ultraviolet spectra. This type of colour vision is referred to as tetra-chromatic because the four different pigments enable the perception of four different colours. Vertebrates that evolved from this early ancestor, such as fish, birds and reptiles, therefore also possess tetra-chromatic vision. Diurnal* fish that live in shallow waters and are exposed to light, for example salmon, still have the ability to distinguish four colours. Similarly, modern-day reptiles and birds, such as crocodiles or sparrows, also have tetra-chromatic vision. 

Mammals evolved from reptiles approximately 200 Mya in an era dominated by the most famous reptiles, the dinosaurs. In order to avoid predation, scientists believe that our mammalian ancestors were nocturnal and hid during the day. In the darkness of the night there was no need for colour perception. Thus, early mammals must have had tetra-chromatic vision but over time two of the four pigments degenerated. Consequently, mammals lost their ability of distinguishing four colours and their vision became di-chromatic. This is why the great majority of modern-day mammals, such as our beloved cats and dogs, can only see two colours. 

Dinosaurs extinguished 65 Mya and so the lower predatory pressure permitted mammals to expand their habitats and acquire diurnal* habits. During that time, primates evolved an extra pigment which enabled them to distinguish blue green and red. These primates therefore had tri-chromatic vision. Scientists believe that the evolution of tri-chromatic vision in primates allowed them to detect ripe fruits and suitable leaves to eat. Just as the other primates, most humans can distinguish three colours: blue green and red. However, there are exceptions to this rule. In capuchin monkeys, for instance, only the females have tri-chromatic vision. In humans, not being able to distinguish three colours is called “colour blindness” or “colour vision deficiency”. This is quite common and occurs in 8% of men and 0.5% of women in the world.

Why do we have a mixture of di- and tri-chromatic vision in some primates?

Primates are social animals and live in groups. It is believed that tri-chromatic vision in some female primates makes them better at locating areas with ripe fruits and edible leaves. These females can then lead the rest of the group towards the food source. Di-chromatic individuals, on the other hand, are better at distinguishing patterns and hence quicker at detecting predators with camouflage patterns. These individuals may be responsible for raising the alarm to alert the rest of the group. Therefore, in a social context, having a mixed group of individuals with both tri- and di-chromatic vision may be more beneficial than a group with uniform vision.

Could it be possible that the same is true for humans? Maybe “colour blind” individuals in pre-historic times were responsible for detecting approaching predators, thus protecting the group.

To understand the text you might need the following concepts:

What is the retina?
Is a region of the eye normally located at the outermost layer ocular globe. This region contains the cells responsible from sensing the light from the outside. It also connects these cells with the neurons that will transmit the information to the visual areas of the brain.(1)

What are vertebrates?
Vertebrates are a subphylum of animals that include jawless fish, jawed vertebrates, tetrapods (reptiles, amphibians, birds and mammals) and fish with bones. They are characterised for having a back bone amongst other features.(2) 

What are diurnal animals?
Diurnal animals are those that remain active during the day and are inactive or sleeping durin thte night. 

Further reading and references:

  1. https://www.britannica.com/science/retina
  2. https://www.britannica.com/animal/vertebrate

Borges, Rui, Warren E. Johnson, Stephen J. O’Brien, Cidália Gomes, Christopher P. Heesy, and Agostinho Antunes. 2018. “Adaptive Genomic Evolution of Opsins Reveals That Early Mammals Flourished in Nocturnal Environments.” BMC Genomics 19(1):121.
Bowmaker, James K. 1998. “Evolution of Colour Vision in Vertebrates.” Eye 12(3):541–47.
Carvalho, Livia S., Daniel M. A. Pessoa, Jessica K. Mountford, Wayne I. L. Davies, and David M. Hunt. 2017. “The Genetic and Evolutionary Drives behind Primate Color Vision.” Frontiers in Ecology and Evolution 5.
Jacobs, Gerald H. 2009. “Evolution of Colour Vision in Mammals.” Philosophical Transactions of the Royal Society B: Biological Sciences 364(1531):2957–67.
Lucas, Peter W., Nathaniel J. Dominy, Pablo Riba‐Hernandez, Kathryn E. Stoner, Nayuta Yamashita, Esteban Lorí- Calderön, Wanda Petersen‐Pereira, Yahaira Rojas‐DurÁN, Ruth Salas‐Pena, Silvia Solis‐Madrigal, Daniel Osorio, and Brian W. Darvell. 2003. “Evolution and Function of Routine Trichromatic Vision in Primates.” Evolution 57(11):2636–43.
Saito, Atsuko, Akichika Mikami, Takayuki Hosokawa, and Toshikazu Hasegawa. 2006. “Advantage of Dichromats over Trichromats in Discrimination of Color-Camouflaged Stimuli in Humans.” Perceptual and Motor Skills 102(1):3–12.

Coconut, a tasty testimony of human globalisation

The coconut palm, Cocos nucifera, is a tropical plant that produces coconuts as a way of reproduction and dispersion towards new areas. Coconuts have the peculiarity of being able to float in sea water. This ability allows coconuts to reach surrounding areas and islands where they germinate and grow into new palms.
The coconut palm is a common crop cultivated across tropical countries. Coconuts are used for food and drink purposes as well as a source of oil, charcoal and fibre worldwide. But, did coconuts spread through all these countries transported by the sea? A genetic study on coconut palms has shown that this is not the case. Coconut palm global expansion is, rather, a result of human movements and trade routes that date from pre-Columbian era.

The coconut palm originated on the Pacific and Indian ocean basins where humans began to cultivate it separately in each of these regions. Over time, the separated cultivation of this plant gave rise to two coconut palm sub-populations with distinctive features, named the Pacific and Indian sub-populations. The distinctiveness of both sub-populations can be observed in the shape of the fruit or the height of the palm but also on the genes. These genetic footprints, together with other anthropological evidences, have allowed scientists to trace the routes by which this plant species acquired its pantropical distribution (Figure below).
The Pacific coconut palm sub-population originated in Southeast Asia and had its main cultivation centres from the Malay peninsula to New Guinea. This sub-population spread towards the east expanding to eastern Polynesia. Interestingly, this sub-population reached the Pacific coast of Latin America 2,250 years ago, likely carried by pre-Columbian Austronesian* seafarers from the Philippines (Figure below). 
The population originating on the Indian ocean was cultivated around 2,500-3,000 years ago with its main cultivation centres in the southern periphery of India including the islands of Sri Lanka, Maldives and Laccadives (Figure below). We have for example historical records that show  the use of coconut in ancient Ayurvedic medicine texts. 
The Indian sub-population initially dominated the Indian ocean basin area. However, the Pacific sub-population was brought into this region, as well as in the east coast of Africa, first through Austronesian trade routes and later through Arab trade routes (Figure below). Archaeobotanical records from the Tanzanian Pemba island for instance demonstrate the importance of coconuts from the 700 to the 1,500 CE in the food culture.
The Indian sub-population, meanwhile, remained confined to the Indian ocean basin. But later on, during European colonial era, its expansion range was extended. Portuguese colonisers brought this coconut palm sub-population to West Africa, Brazil and the Caribbean. In Mexico, instead, only the Pacific sub-population is present which is likely related to Spanish colonisers importing plants from Solomon Islands (Figure below).

But why was this plant carried around for cultivation? It Is likely that it enabled the establishment of maritim trade routes by providing a way of transporting food but also importantly drinking water.
So, next time you sprinkle some coconut on a cake, use coconut fibre on your potted plants or wear coconut jewelery, think that this plant is a living witness of human globalisation history.

Figure taken from: Gunn, Bee F., Luc Baudouin, and Kenneth M. Olsen. 2011. “Independent Origins of Cultivated Coconut (Cocos Nucifera L.) in the Old World Tropics”. PLoS ONE 6(6):e21143.

To understand the text you may need the following concepts:
Who are the Austronesians?
The people that inhabited the areas of remote Oceania and were Austronesian speakers. This people came from a prehistoric migration from Taiwan around 3,000 to 1,500 BCE and concluded the last stage of Neolithic human expansion.(1)

Further reading and references:
Gunn, Bee F., Luc Baudouin, and Kenneth M. Olsen. 2011. “Independent Origins of Cultivated Coconut (Cocos Nucifera L.) in the Old World Tropics” edited by P. K. Ingvarsson. PLoS ONE 6(6):e21143.
(1) Chang, C.S., Liu, H.L., Moncada, X., Seelenfreund, A., Seelenfreund, D. and Chung, K.F., 2015. A holistic picture of Austronesian migrations revealed by phylogeography of Pacific paper mulberry. Proceedings of the National Academy of Sciences112(44), pp.13537-13542. 

How do plants sense the spring?

When spring arrives, plants start to sprout and soon delight us with their colourful flowers and pleasant scents. But how do plants know that it is time to bloom? 

Plants have a biological clock, also known as the circadian clock, which coordinates key physiological processes that occur throughout the day and through the seasons. Such processes include petal opening, leaf movement or flowering time. This biological clock is by default determined internally. However, its rhythm is modified by external signals such as light and temperature. In this way, plants are able to set the clock time to the environment in response to external cues. By doing that, plants can, for example, adapt to the lengthening of days from winter to summer.

There is not only one circadian clock in every plant but a multitude of them that need to be synchronised to ensure the correct growth, development and reproduction of the plant as a whole. At the tiniest level, each cell* within the plant has its own clock. These cellular clocks are synchronised at a tissue* level, which constitutes the association of cells with similar functionalities. Plant organs*, such as leaves, shoots and roots, are each formed by different tissues themselves. So, all the tissues within an organ are also synchronised. As each plant organ fulfils a different role within the plant, the clock of each organ will display different sensitivities to the various environmental cues. For instance, light will strongly influence leaves, whose major function is to transform the light into energy via photosynthesis*. Roots, however, will be less affected by light, as their major function is to absorb water and nutrients from the soil. All organ clocks will be synchronised between themselves but also with the ecosystem that is inhabited by the plant. For instance, many plants rely on pollinators* for their reproduction. Hence, plants will coordinate their flowering time and production of scents to attract pollinators when these are more active, for example in spring. 

This is why, when temperature rises and days are getting longer, we can admire the beauty of daffodils, snow drops and hyacinths that once again wave us with their joyful colours and fragrant smells.

To understand the text you may need the following concepts:
What is a cell?
Cells are the building blocks of all living organisms. If we would compare a multi-cellular organism to a house, cells would be the bricks.(1)
What is a tissue?
Tissues are, in multi-cellular organisms, the organisation level constituted by groups of cells with similar structure and function. If we would compare a multi-cellular organism to a house, we would consider as separate tissues the walls, floors and ceilings.(2)
What is an organ?
Organs are the next level of organisation above the tissue level. These are composed of different tissues and have a specific function within the multi-cellular organism. If we would compare a multi-cellular organism to a house, an organ would be each of the rooms which has a specific function, for example the kitchen for cooking or the bedroom for sleeping.(3)
What is photosynthesis?
Photosynthesis is the process by which plants and other organisms use light to produce energy. As a consequence plants take up CO2 from the atmosphere and secret oxygen. Hence, we recognise plants as very important in fighting climate change.(4)
What are pollinators?
Pollinators are animals, like the well-known bees, that aid plants to reproduce. They visit flowers attracted by their colours and scent. When pollinators visit flowers, pollen grains attach to their surface. Pollen are male reproductive cells of plants that need to be transported to other flowers for reproduction. So, when the same pollinator visits another flower, the pollen grains carried on its surface will be deposited and will fertilise the female part of the visited flower. (5)

References and further reading:
– Greenwood, Mark, and James CW Locke. 2020. “The Circadian Clock Coordinates Plant Development through Specificity at the Tissue and Cellular Level.” Current Opinion in Plant Biology 53:65–72.
– McClung, C. Robertson. 2006. “Plant Circadian Rhythms.” The Plant Cell 18(4):792–803.
1. https://www.nature.com/scitable/topicpage/what-is-a-cell-14023083/
2. https://www.britannica.com/science/tissue
3. https://www.britannica.com/science/organ-biology
4. https://www.livescience.com/51720-photosynthesis.html
5. https://ento.psu.edu/pollinators/resources-and-outreach/what-are-pollinators-and-why-do-we-need-them

Ants as caterpillar farmers

Ants are fascinating social insects with recognised farming abilities. This has recently been shown in the interaction between the ant species Camponotus compressus and the gram blue lycaenid* butterfly Euchrysops cnejus while coexisting on the cowpea plant Vigna unguiculate. Upon detection of butterfly larvae*, ants construct a shelter at the base of the plant. Larvae, when ready to pupate*, move down from the plant to the shelter where they will stay until reaching adulthood and becoming butterflies. During this period, ants attend pupae and may play an important role in protecting them against natural enemies. Meanwhile, ants benefit from the sugary secretions produced by the pupa. Here, nature presents us with another beautiful example of mutualism* between these two insect species. 
This information has been obtained from the following Research article. For more detailed information please refer to it.
A single lycaenid caterpillar gets an ant-constructed shelter and uninterrupted ant attendance by Priya Aradhya Ekka & Neelkamal Rastogi. Entomologia Experimentalis et Applicata (2019).

To understand the text you may need the following concepts:
*What is mutualism? Mutualism is a type of ecological interaction between individuals of different species in which all the organisms involved obtain a net benefit.(1)
*What is a lycaenid butterfly? These are butterflies that belong to the family Lycaenidae, the second-largest family of butterflies.(2)
*What is a butterfly larva and pupa? The butterfly life cycle encompasses different developmental stages which progress from egg to larva, pupa until adult butterfly.
The initial developmental stage is the egg from which a larva, also known as caterpillar for moths and butterflies, emerges. This larva is the feeding stage as it will solely eat to grow and store nutrients. Once the larva is fully grown, it enters the following phase known as pupation. Pupae, also named chrysalis in the case of butterflies, will remain quiet while the whole body changes structure to become an adult butterfly, process known as metamorphosis. The adult butterfly will then reproduce and lay new eggs closing the life cycle.(3)

  1. J.N. Holland, J.L. Bronstein, in Encyclopedia of Ecology, 2008
  2. Pierce, N. et al. “The Ecology and Evolution of Ant Association in the Lycaenidae (Lepidoptera)” ( 2002). Annual Review of Entomology 47 (1): 259-267
  3. The Academy of Natural Sciences of Drexel University (https://ansp.org/exhibits/online-exhibits/butterflies/lifecycle/)