Humans are Mammals, Primates
Humans are descendants of mammals who appeared about 250 million years ago
when the major land masses were connected. Mammals evolved into a great variety
of creatures who had warm bodies, gave birth to live babies and fed them milk.
Mothers assumed a greater role in assuring the survival of babies and social
organization became critical for many mammals to survive. Humans are primates
who, according to genetic analysis, diverged from other placental mammals about
90 million years ago. The oldest known fossil primates are from the Eocene epoch
55 million years ago. Tavaré et al suggested that only 7% of all primate species
that have ever existed are known from fossils. One of the mammalian-primate
lineages, "hominids" are included in the superfamily of all apes, the
Hominoidea. Some hominids became human in a series of transformations that led
to our current form sometime in the last 2- 300,000 years.
Fossil evidence suggests that modern humans are descendants of black Africans who migrated from Africa into Asia and Europe. Ledgard described visiting the real Garden of Eden in Kenya: "An hour’s drive and a 600-metre drop in altitude from Nairobi is Olorgesailie, a Lower Palaeolithic archaeological site on the floor of the Rift Valley in Kenya. It is blisteringly hot. Nothing moves in the heat of the day except dust, gathering into twisters. It might be the closest we have to the Garden of Eden. From the campsite it is possible to make out the outline of the prehistoric lake which once flooded the plain in soapy water. Hominids lived here for 900,000 years. They made handaxes which they used to butcher the hippos, zebras and baboons they hunted and scavenged. The Kenyan anthropologist Louis Leakey uncovered a Homo erectus skull here in the 1940s; the brain cavity was disappointingly small. There must have been grunts, gestures with stones, blood, the sky blotted with vultures, ape children kept back in the darkness. The sense of space here is immense. So too is the sense of known time, hominid time, known at first in the way a beast knows time, in light and darkness, but conscious all the same. The night sky is black lacquered. There are shooting stars. Sometimes there is the sound of hyenas."
Humans are Primates
The tendency in paleoanthropology is to discover more and older fossil hominids with transitional characteristics. The timing and location of this transformation will never be known with certainly. Lucy was the first bipedal Australopithecus Afarensis skeleton found in the mid-1970s with an estimated age of 3.2 million years. An older, taller hominid skeleton was found in the Rift Valley, Ethiopia, 2005 just north of Hadar where Lucy was discovered. Nicknamed 'Big Man', the 3.58-million-year-old male skeleton suggested a running and walking biped nearly 2 metres tall. His scapula which anchors the shoulder muscles, is similar to that of a modern human suggesting a move from arboreal life. The oldest Ethiopian hominid, Ardipithecus ramidus (4.4 million years) had more ape-like arms and feet. In the interim periods other hominins appear and disappear.
Rech et al summarized recent history: "About 200,000 years ago, anatomically modern humans (that is, humans with skeletons similar to those of present-day humans) appeared in Africa. At that time, as well as later when modern humans appeared in Eurasia, other ‘archaic’ hominins were already present in Eurasia. In Europe and western Asia, hominins defined as Neanderthals on the basis of their skeletal morphology lived from at least 230,000 years ago before disappearing from the fossil record about 30,000 years ago. In eastern Asia, no consensus exists about which groups were present. Until at least 17,000 years ago, Homo floresiensis, a short-statured hominin that seems to represent an early divergence from the lineage leading to present-day humans was present on the island of Flores in Indonesia and possibly elsewhere. On the Eurasian mainland there existed at least two forms of archaic hominins in the Upper Pleistocene: a western Eurasian form with morphological features that are commonly used to define them as Neanderthals, and an eastern form to which the Denisova individuals belong."
A Nature Editorial comments on new evidence of early Homo sapiens: “The exact place and time that our species emerged remains obscure because the fossil record is limited and the chronological age of many key specimens remains uncertain. Previous fossil evidence has placed the emergence of modern human biology in eastern Africa around 200,000 years ago. In this issue of Nature, Jean-Jaques Hublin and colleagues report new human fossils from Jebel Irhoud, Morocco; their work is accompanied by a separate report on the dating of the fossils by Shannon McPherron and colleagues. Together they report remains dating back 300,000–350,000 years. They identify numerous features, including a facial, mandibular and dental morphology, that align the material with early or recent modern humans. They also identified more primitive neurocranial and endocranial morphology. Collectively, the researchers believe that this mosaic of features displayed by the Jebel Irhoud hominins assigns them to the earliest evolutionary phase of Homo sapiens. Both papers suggest that the evolutionary processes behind the emergence of modern humans were not confined to sub-Saharan Africa.”
(Editorial commenting on N. Hublin et al . New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens. Nature 546, 289–292 (08 June 2017)