Spectrum of Camel Evolution and its Impact on Ancient Human Migration

The Evolutionary history and domestication of Camels are largely unexplored because of the lack of well dated early archaeological records. However, limited records suggest that domestication of Camels likely happened in the late second millennium BCE. Over the time, camels have helped human for their basic needs like meat, milk, wool, dung to long routes transportation. This multifaceted animal has helped the mankind to connect through continents and in trade majorly through the Silk route. In India, both dromedary and Bactrian camels are found and their habitat is entirely different from each other, dromedaries inhabit in hot deserts and Bactrians are found mostly in cold places (Nubra Valley, Ladakh). Fewer studies on Indian dromedaries have been conducted but no such studies are done on Bactrian camels. It is needed to study the genetics of Bactrian camels to find out their genetic affinity and evolutionary history with other Bactrians found in different parts of the world. Furthermore, parallel studies on humans and Bactrian camel are required to understand the co-evolution and migration pattern of humans during their dispersal in different time periods.


Milieu
Camels have survived and adapted to earth"s most crucial changes and also played a pivotal role in human survival since ancient times as they facilitate trading as well as cultural exchange.
Moreover, camels are considered as "the most adaptable travelers" considering their time to time adaptations with the crucial changes in the climate. There are three extant species of the camel viz. one-humped dromedary (~94% of the total camel population), two-humped Bactrian camel (6% of the camel population) (Wu et al., 2014) and the Wild Bactrian camel which is considered as a separate species and has been included in the list of critically endangered species (Kaczensky et al., 2014). The most accepted classification of the family Camelidae is presented in the figure 1.  (Wu et al., 2014) In quest of camel as most adaptive life-form it becomes pertinent to understand the chronological evolution of camels that may also provide some insight towards the palaeo-geography and climate; their impact on phenotypes and migration pattern of other life form especially humans.

Adaptive Camel Evolution in Chronological Epoch
Origin of camels traces back to the Eocene period about 40-45 million years ago in North America (P. A. . Of the entire Cenozoic, early Eocene epoch had the highest mean annual temperatures and high ice free precipitation. Protylopus, the most primitive camelid which occurred in the Eocene, having no hump and were of the size of a hare (Rybczynski et al., 2013). During this period, a significant plate boundary rearrangement occurred due to the tectonic shifts that resulted in major effects on atmospheric temperature. A major event of global cooling was observed during the middle Eocene due to the creation of circum Antarctic current created during separation of Antarctica and Australia (P. A. Burger, Ciani, & Faye, 2019). The end of Eocene was marked by lower temperatures and increased seasonality that resulted in alteration in the type of vegetation to savanna-like and increased body size of mammals. Further, camelids evolved and Proebotherium, a camelid found in the middle of Oligocene and Miocene around 35 million years ago, was about the size of a goat having long neck that would helped them in approaching comparatively higher vegetation with no hump like the previous ones.
Further, evolution of camelids took place into procamelus which survived from Oligocene to Miocene. This was the period when cooling of the earth took place and has resulted in many changes like replacement of dry savannah vegetation to steppe.
Owing to these changes in the environment during the end of Miocene and the beginning of Pliocene, camelids evolved into much larger sized body with long neck than their predecessors.
Around 25 million years ago, due to the bridging of North and South America, many camelids It is believed that the Paracamelus has evolved into the modern Camelus (Yam & Khomeiri, 2015). One-humped modern dromedary is supposed to be evolved from the two-humped bactrian species, i.e., Camelus bactrianus having thick coat that can withstand very low temperature. It has been reported that around 2 million years ago, Bactrian somewhat similar to modern bactrian, having a height of about 7 feet might had evolved from the Procamelus/ Paracamelus camelids that had moved into the arctic regions of Asia through the Bering Strait (P. A. Burger et al., 2019). The evidence of their movement can be evidences by fossils of giant camels e.g.

Presence of Camels, their domestication and putative impact on Human Migration
The most remarkable characteristic of the camels is their adaptation capability favored by natural selection, they have undergone deep historical events of climatic changes (glacial periods to their survival in arid conditions). Their unique physiological and morphological characters to provide the essential products like milk, meat, transportation and dung under extreme conditions made them ideal as military animals, incense roads and domestication (Bulliet, 1990;. The domestication of Bactrian camel was estimated to be near the beginning of third millennium (Bulliet, 1990) whereas domestication of dromedaries were estimated in the late second millennium (Uerpmann & Uerpmann, 2002). The domestic camels were important being multi-purpose in civilizations of the old world and even today, evident by mitochondrial genetic profile of the camels that reflects their movements and extreme trade (P. A. Burger et al., 2019).
However, the exact region for the domestication remains uncertain but with the advent of high throughput sequencing techniques, the researchers have been able to detangle the mystery by analyzing the data from the domesticated and wild Bactrian camels from Asia. The study supported that the origin of domestic Bactrian camels from Central Asia which later on migrated eastwardly to Mongolia (Ming et al., 2020). other livestock species (Edwards, Baird, & MacHugh, 2007;Fages et al., 2019;Ming et al., 2020). Since no such study has been carried out on Indian Bactrian/dromedary camel, extensive sampling and genome wide analysis is needed to be conducted to fill the void in the literature. It will deepen our knowledge about the genetic diversity and genetic architecture of camels present in India.
Both Bactarian and Dromedary camels have been found in India but have occupied different habitats; where Bactarian species are found in cold desert, dromedaries are inhabitants of hot desert. The species of single hump dromedary camels are found in western desert of India (Rajasthan and Gujarat) are Bikaneri, Jaisalmeri, Kutchi and Mewari (Mehta, Mishra, & Sahani, 2006). Double humped Bactarian species of Camel is only found in cold arid region of Nubra valley of Ladakh. These species were the main mode of transport via silk route due to their adaptability to lowest temperatures, scarce feeding and water availability (Gazi, ul Nabi, & Ahmed, 2013) and it is believed that these belong to group of camels which left here in India due to the closure of Indian Tibet border due to the Chinese aggression (Banerjee, Joshi, Ganai, & Vijh, 2012). It has been reported that around 220 Bactarian camels are found in Ladakh which is quite a less number (Gazi et al., 2013). Extensive research has been performed on the Indian dromedaries like genetic characterization, nutrient utilization, hormonal patterns, reproductive features etc. which has led to their better breeding and management (Agarwal, Khanna, Agarwal, & Dwaraknath, 1986;Agarwal, Khanna, Agarwal, & Dwaraknath, 1987;Gautam, Mehta, Gahlot, & Gautam, 2004;Khanna, Tandon, & Rai, 1990;Mehta, 2013;Mehta et al., 2006;Nagpal & Patil, 2013;Nagpal, Rai, & Khanna, 1993). On the contrary, very few studies have been conducted on the Indian Bactrian camel when compared to the dromedaries. In order to observe the genetic diversity we tried to perceive the genetic data from different geographical locations of camels in different databases and observed that most of the studies are focused on whole mtDNA genome, very less on Y chromosome and recently researchers were able to generate whole genome data from limited locations. The phylogentic tree from whole mtDNA of camel from different geograhical was drawn. It has been observed that scant work has been carried out on Indian camels at molecular level. The study conducted on Indian Bactrian camel, based on microsatellite loci reported quite a good level of heterozygosity in them that could be exploited by implementation of strategies for conservation and by checking inbreeding (Banerjee et al., 2012). Moreover, other studies like morphometeric characterization, analysis on reproductive status, hematological, physio-biochemical analysis of Bactrians in India have also been done but needed to be explored for the proper camel husbandary as well as management of their health and also for the future research projects (Gazi et al., 2013;Lamo et al., 2020;Vyas et al., 2015).
As discussed earlier, it has been reported that the Bactrians in India are the left out population but there are no concrete evidences at the molecular level which could detangle and confirm the ancestory and evolutionary history of these double hump camelids found in India. By implying different genetic studies like exploring mitochondrial DNA, microsatellite markers, whole genome sequencing, we can determine their origin and genetic diversity which will help us in understanding the pattern of gene flow and phylogeography of the Indian double humps with that of the other Bactrians found in different geographical locations. Further, the genetic data for the different species of camels in India required to be analyzed and comparision with other populations of camels present in different geographical locations is essential to observe the diversity of Indian camels (both dormedry and bactrian) at different time scale. It will be interesting to explore the direction of selection of genes that aided this living being to survive in extreme conditions. Additionally, by comparing the migration pattern of camels along with humans can also aid in tracing the missing link of human gene flow especially the South Asian populations where these beings are believed to be exploited for migration due to their multiple charactertics and high endurance towards extreme conditions. This will also help in knowing the clearer relationship between camel and human, their domestication and migrating from Africa to other parts of the globe. Effective population size of Bactrian camels was reduced upto 70% around 100,000-20,000 Years Before Present (P. A. , it is also been found that during the same time period a diverged population from modern humans, who have contributed to ancestoral human populations (Kuhlwilm et al., 2016) could have utilized these camels as source for survival. To have a deeper insight into this hypothesis, ancient DNA/fossil studies of double humped camels and humans along with the migration routes followed by the early humans migrating "out of Africa" is needed to be explored and studied extensively.
Understanding human evolution in context to the human migration could not be possible without understanding the migration patterns of the domesticating animals, since domesticated animals and human share common migration patterns because of their interdependency. A study carried on mtDNA of 781 ancient and mordern Sus speciemens revealed, human mediated dispersal of Sulawesi warty pig (Sus celebensis) and domestic pig (Sus scrofa) through Southeast Asia into Oceania (Larson et al., 2007). It because of the domestication of animals, that led humans to change from hunter-gatherer into sedentarty agriculture lifestyle and also led to the development of societies (Frantz, Bradley, Larson, & Orlando, 2020). Dispersal of dogs back 11,000 years ago was also supposed to have took place along with humans. Humans have domesticated dogs and took them to different parts of the world during migration (Callaway, 2020). These studies proved that humans and domestic aniamls have something common in their migration pattterns.
Taking these hypothesis into consideration, it becomes pertinent to explore the dispersal of camel population which may help us in understanding the pattern of human migration to some extent.