Why are Apples Red?
Now we have had a kōrerorero ( Maori: discussion) on social media here is my take on why wild apples are mainly red.
The little neo-apple started as a seperate species 65 million years ago with a major gene duplication and rearrangement from the effects of the asteroid hit causing the Yucatan crater off Mexico ( which wiped out the dinosaurs and caused a major climate disruption affecting all plants and animals)
See seperate post. This neo apple would probably have looked more like a hawthorn with bunches of long-stalked fruits not much bigger than peas. Smaller than crabapples today.
About 40 million years ago, the Indian subcontinent, which had detached from East Africa, crashed into the great northern land mass of Asia, this thrust successively drove up the Himalayas, Kunlun mountains, the Pamirs and the Tian Shan. India is still moving northward at several centimetres a year. The Tian Shan, the last of the ripples to form , began to emerge as a distinct geological feature about 12 million years ago, and it is still lengthening and rising, in places, up to 1.5 centimetres per year. The Tian Shan peaks are well above the snow line and form a great mountain range about 1,600 kilometres miles long and 640 kilometres in depth, running from what is now China in the east to Uzbekistan in the west with about 20 parallel ridges.
The vast mass of the Indian Ocean to the south, through the constant warm monsoons warms the land mass well into the interior of China , and has kept the region including Tian Shan glacier free although climates have varied over the Millenia with expansion and contraction of deserts especially the Gobi desert to the north of a temperate belt.
As a result of this warming by the Indian Ocean over millions of years, the Tian Shan has become a refuge for the sustained evolution of plants and animals. Unlike most of Western Europe which suffered the effects ice scraping clean the land as well as the cold. The last Ice age being only 10,000 years ago. The mountains are constantly exposing new geological facies, disturbing existing drainage systems, wrecking existing patterns of vegetation and yet at the same time exposing fresh soil sites for exploitation by incoming seedlings.
About 10 million to 12 million years ago there must have existed somewhere in what is now central China, roughly in the area of the current provinces of Xi'an and Shaanxi, a Tertiary temperate forest where the neo apple grew ( The Tertiary Period began about 66 million years ago with a mass extinction including the dinosaurs after the Chicxulub asteroid hit Earth, and ended when the ice ages of the Quaternary Period began, about 2.6 million years ago.)
This forest moved into this geologically restless but fecund environment as the climate changed and with it moved ancestors of thousands of present-day plants, including those of many important crops and, alongside them, many animals including bear, deer and wild pig and birds. from Xi,an along the Ganzu corridor which remained temperate despite the changing climates.
In the forest, along with these neo-apples would be other fruits that have developed into apricots, pears, pomegranates, figs, cherries, and mulberries.
The apples were firmly attached long stems and brightly coloured with carotenoids and anthrocyanins to attract birds with movement and colour as crab apples still are as birds have a wide range of colour perception ( wider than mammals into the red ends of the light spectrum). A carrier-bird candidate is the azure-winged magpie (Cyanopica cyanus), which now occurs in 2 seperate lands with virtually identical populations at both ends of the original transcontinental forest, in southern Europe and eastern China. The apple pips of these small but evolving neo apples may have been carried unharmed in a bird's crop.
Birds, of course, eat all manner of fruit. Most birds eat seeds—a dietary feature not conducive either to the selection or spread of a modern large apple bearing fruit tree. . As the fruit sweeten and enlarge, the apples are eviscerated by birds, but the seeds are frequently left in the empty shell of the pome as the birds become saieted. For some birds the reason is that apple seeds contain cyanoglycosides, which are repellent.
So there must be other drivers at work for seed dispersal. The most likely candidate is the Tian Shan bear which have an impressively catholic diet—from wild rhubarb in the spring, through bee grubs and honey in the late summer, yes bears do have a sweet tooth, as noted by A. A. Milne noted in Winnie the Pooh!
And to apples in the autumn when bears just prior to hibernation select the larger and sweeter fruits of the neo-apples.
Fleshy fruits are usually adaptations for animal dispersal (endozoochory); large fruits are usually adaptations for large mammalian dispersal.
As the bears move the faeces containing viable pips are deposited widely over the Tian Shan in the autumn and in a fertile mix of nitrogen-rich compost, and some of the pips will have the right conditions to germinate to form a new tree. Conditions appear to be firstly separated from the apple flesh exposed to sunlight and post cold conditioning - stratification.
However why are the apples still red, a characteristic displayed to attract birds? Bears are said to be dichromic and see only Violet to Green in the visual spectrum not on to Red wavelengths . And therefore would not have an evolutionary advantage and so likely to drop out under selection pressure.
Before discussing this further it’s worth looking at the physiology of colour vision. Most animals including bears are Dichromic with two types of cones in the retina the S cone ( S stands for short and id /tuned to Violet 380 nm at the short wavelength of the spectrum and L cones that are tuned to longer wavelength of Green about 500nm.
Animals with dichromatic vision can see colour, but their ability to distinguish between longer wavelength colours such as red is reduced compared to those with trichromatic vision such as mammals including humans who have 3 different cones. An ‘M’ ( Medium) cone has its maximum sensitivity near green 480nm and a an L cone shifted into the red at 550nm . This type of colour vision provides for a great range of colour discrimination, allowing the brain to compare and contrast the signals from all three types of cones to create a detailed picture of the colour being viewed. Red-Green colourblind people report this as seeing greys.
So back to bears. Experiments have shown that although bears see greys being intelligent they can quickly distinguish between green and red foods! Mystery solved. The red colour in apples, until recently when humans entered the picture and have breed many apples of size, colour and flavours, remained a useful evolutionary characteristic.
Other large omnivores and herbivores may also have helped such as deer and horses with seeds trapped to the hooves by mud. Of course this corridor was not only used by plants and animals but also by humans and their beasts of burden, and whilst trading brought the apple to the West
Notes and ideas for this article gained from several sources but thanks especially to The Story of the Apple by Barrie Juniper and David Mabberley.